[llvm-exegesis] Guard __builtin_thread_pointer use with __has_builtin

While Clang targets have supported __builtin_thread_pointer for a very
long time (e.g., 2007 for AArch32, 2015 for AArch64), for some GCC
ports, the support is very new (11.0 for x86[1], while we need to
support GCC 7), and many ports haven't implemented
__builtin_thread_pointer yet (m68k, powerpc, etc).

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96955