[X86] Selective relocation relaxation for +tagged-globals

For tagged-globals, we only need to disable relaxation for globals that
we actually tag.  With this patch function pointer relocations, which
we do not instrument, can be relaxed.

This patch also makes tagged-globals work properly with LTO, as
-Wa,-mrelax-relocations=no doesn't work with LTO.

Reviewed By: pcc

Differential Revision: https://reviews.llvm.org/D113220