[compiler-rt] [compiler-rt][GWP-ASan] Use no-op backtrace if execinfo.h missing (PR #201266)

[Brian Cain via llvm-commits]

androm3da wrote:

> __has_include could be simpler

Rebased and pushed a fixup commit with the suggested change.

https://github.com/llvm/llvm-project/pull/201266