[llvm-exegesis] Guard `__builtin_thread_pointer` behind a configure check

Due to arguably a bug in GCC[0], using `__has_builtin` is not sufficient to check whether `__builtin_thread_pointer` can actually be compiled by GCC. This makes it impossible to compile LLVM with `llvm-exegesis` enabled with e.g. GCC 10 as it does have the builtin, but no implementation for architectures such as x86.

This patch works around this issue by making it a cmake configure check whether the builtin can be compiled and used, rather than relying on the broken preprocessor macro.

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96952, demonstration: https://godbolt.org/z/9z5nWM6Ef

Differential Revision: https://reviews.llvm.org/D155828