[HWASan] Use page aliasing on x86_64.

Userspace page aliasing allows us to use middle pointer bits for tags
without untagging them before syscalls or accesses.  This should enable
easier experimentation with HWASan on x86_64 platforms.

Currently stack, global, and secondary heap tagging are unsupported.
Only primary heap allocations get tagged.

Note that aliasing mode will not work properly in the presence of
fork(), since heap memory will be shared between the parent and child
processes.  This mode is non-ideal; we expect Intel LAM to enable full
HWASan support on x86_64 in the future.

Reviewed By: vitalybuka, eugenis

Differential Revision: https://reviews.llvm.org/D98875