[x86] Implement a tagged-globals backend feature.

The feature tells the backend to allow tags in the upper bits of global
variable addresses.  These tags will be ignored by upcoming CPUs with
the Intel LAM feature but may be used in instrumentation passes (e.g.,
HWASan).

This patch implements the feature by using @GOTPCREL relocations instead
of direct references to the locally defined global.  Thus the full
tagged address can be loaded by a single instruction:
  movq global@GOTPCREL(%rip), %rax

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D111343

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