[BOLT-AArch64] Fix BOLT build on AArch64

Summary:
Whenever building BOLT in an AArch64 box, we need to make sure
we do not run tests that are excluse for x86. This diff also adds a tag
for expensive tests, so the user can disable them, which is useful when
using a memory-constrained machine to run BOLT tests. It also removes
ifdefs that caused BOLT to behave diferently when running in a non-x86
host. Finally, it changes a case where we depended on updated libstdc++
implementation for insert to make the codebase more friendly with boxes
that do not have the newer version of the lib.

(cherry picked from FBD7625715)