[compiler-rt] [llvm] [X86][BF16] Add libcall for F80 -> BF16 (PR #109116)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-cmake-x86_64-avx512-win` running on `avx512-intel64-win` while building `compiler-rt,llvm` at step 5 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/81/builds/1960

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build stage 1) failure: 'ninja' (failure)
...
[110/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\umodsi3.c.obj
[111/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\udivti3.c.obj
[112/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\udivsi3.c.obj
[113/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\umoddi3.c.obj
[114/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\udivmodti4.c.obj
[115/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\udivmoddi4.c.obj
[116/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\mulsf3.c.obj
[117/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\truncdfhf2.c.obj
[118/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\truncsfhf2.c.obj
[119/130] Building C object CMakeFiles\clang_rt.builtins-x86_64.dir\truncxfbf2.c.obj
FAILED: CMakeFiles/clang_rt.builtins-x86_64.dir/truncxfbf2.c.obj 
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\stage1\.\bin\clang-cl.exe --target=x86_64-pc-windows-msvc  /nologo   /DWIN32 /D_WINDOWS /Zi /Ob0 /Od /RTC1 -fno-lto /Zl -fno-builtin -DCOMPILER_RT_HAS_FLOAT16 /showIncludes /FoCMakeFiles\clang_rt.builtins-x86_64.dir\truncxfbf2.c.obj /FdCMakeFiles\clang_rt.builtins-x86_64.dir\clang_rt.builtins-x86_64.pdb -c -- D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\truncxfbf2.c
In file included from D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\truncxfbf2.c:11:
In file included from D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc_impl.inc:39:
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(49,2): error: Source should be double precision or quad precision!
   49 | #error Source should be double precision or quad precision!
      |  ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(114,15): error: unknown type name 'src_rep_t'; did you mean 'dst_rep_t'?
  114 | static inline src_rep_t extract_sign_from_src(src_rep_t x) {
      |               ^~~~~~~~~
      |               dst_rep_t
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(99,18): note: 'dst_rep_t' declared here
   99 | typedef uint16_t dst_rep_t;
      |                  ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(114,47): error: unknown type name 'src_rep_t'; did you mean 'dst_rep_t'?
  114 | static inline src_rep_t extract_sign_from_src(src_rep_t x) {
      |                                               ^~~~~~~~~
      |                                               dst_rep_t
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(99,18): note: 'dst_rep_t' declared here
   99 | typedef uint16_t dst_rep_t;
      |                  ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(115,9): error: unknown type name 'src_rep_t'; did you mean 'dst_rep_t'?
  115 |   const src_rep_t srcSignMask = SRC_REP_C(1) << (srcBits - 1);
      |         ^~~~~~~~~
      |         dst_rep_t
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(99,18): note: 'dst_rep_t' declared here
   99 | typedef uint16_t dst_rep_t;
      |                  ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(115,33): error: call to undeclared function 'SRC_REP_C'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  115 |   const src_rep_t srcSignMask = SRC_REP_C(1) << (srcBits - 1);
      |                                 ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(115,50): error: use of undeclared identifier 'srcBits'; did you mean 'dstBits'?
  115 |   const src_rep_t srcSignMask = SRC_REP_C(1) << (srcBits - 1);
      |                                                  ^~~~~~~
      |                                                  dstBits
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(101,18): note: 'dstBits' declared here
  101 | static const int dstBits = sizeof(dst_t) * CHAR_BIT;
      |                  ^
D:\buildbot\llvm-worker\clang-cmake-x86_64-avx512-win\llvm\compiler-rt\lib\builtins\fp_trunc.h(116,32): error: use of undeclared identifier 'srcBits'; did you mean 'dstBits'?

```

</details>

https://github.com/llvm/llvm-project/pull/109116