Remove HAVE_STRERROR

Most systems support strerror_r. For the remaining systems (e.g. MSVC) strerror_s and
strerror can be used as fallbacks. We don't have a supported operating
system/compiler that doesn't provide `strerror`.

Close https://github.com/llvm/llvm-project/issues/62804

https://github.com/flang-compiler/f18/pull/1068 added a fallback
when strerror is unavailable, but I think the code path is dead.

Reviewed By: serge-sans-paille, vzakhari

Differential Revision: https://reviews.llvm.org/D151718