[llvm] RuntimeLibcalls: Add bitset for available libcalls (PR #150869)

[Sergei Barannikov via llvm-commits]

================
@@ -616,6 +618,21 @@ else ()
   endif ()
 endif ()
 
+foreach(i IN ITEMS 8 4)
+  try_compile(SIZEOF_UINTPTR_T_IS_${i}
----------------
s-barannikov wrote:

TL;DR I don't know.

When I wrote this comment I assumed LLVM_NATIVE_ARCH is the host architecutre. It doesn't seem to be the case: it gets its value from LLVM_TARGET_ARCH (unless it equals to "host"), which has the following comment:
```
LLVM_TARGET_ARCH Set target to use for LLVM JIT or use \"host\" for automatic detection.
```
I guess it is not what we need here.

If LLVM_TARGET_ARCH == "host", LLVM_NATIVE_ARCH is set to the value of LLVM_HOST_TRIPLE and the above if/else adjusts it as necessary.
https://llvm.org/docs/HowToCrossCompileLLVM.html says 
```
LLVM_HOST_TRIPLE: Specifies the target triple of the system the built LLVM will run on
```
This does look relevant, but I don't know how to pass it to try_compiler or whether it is passed automatically.


https://github.com/llvm/llvm-project/pull/150869