[compiler-rt] [compiler-rt] Explicitly enable C extensions for profile (PR #110555)

[Raul Tambre via llvm-commits]

tambry wrote:

1. `C_EXTENSIONS` has been supported since CMake 3.1. LLVM requires 3.20.
2. If [CMP0128](https://cmake.org/cmake/help/latest/policy/CMP0128.html) isn't supported then the behaviour remains the same. Compilers with extensions off by default won't have them enabled and won't be able to compile the code. No other relevant changes for LLVM.

CMP0128 is basically a bugfix ([I implemented it](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/6177)).  
But since there's code in the wild depending on the bug it became a policy. For example some projects came to rely on `<LANG>_EXTENSIONS=OFF` actually _not_ disabling extensions in some cases.

https://github.com/llvm/llvm-project/pull/110555