[compiler-rt] [compiler-rt] Allow building builtins.a without a libc (PR #86737)

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

CMAKE_C_COMPILER_WORKS=TRUE doesn't help, sadly. I've tried a lot of cmake cross compilation hacks lately without much luck. I think cmake deprecated and then disabled CMAKE_C_COMPILER_WORKS at some point. I think in principle cross compilation can work, but our scripts have a lot of ad hoc compiler and run style checks like this one which vary in the extent to which they pass cmake flags around. There's a CheckAtomic or similar elsewhere in runtimes which has failed a lot as I try to find some path through our scripts.

I'll drop the stdint.h part, not sure it adds anything.

Compiling compiler-rt builtins by itself is a really useful first step in standing up a sysroot.

https://github.com/llvm/llvm-project/pull/86737