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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Hmm - FWIW, for doing this kind of bootstrap, it's already possible by building the builtins library on its own, standalone from the rest of compiler-rt, by pointing cmake at `compiler-rt/lib/builtins`, usually also setting `CMAKE_C_COMPILER_WORKS=TRUE` and `CMAKE_CXX_COMPILER_WORKS=TRUE` at the same time, to waive some sanity checks.

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