[MTE] Add memtag_globals as a sanitizer target.

MTE globals needs four pieces:
 1. (done) clang/llvm to produce objfiles with MTE globals metadata
 2. (in flight, https://reviews.llvm.org/D152921) lld to process the metadata in the objfiles, and produce information in each DSO describing the global variables that need to be tagged (alongside some other stuff).
 3. (this patch) android to handle the new sanitizer
 4. (not yet sent) bionic's libc/linker to interpret the information in each DSO, and tag the global variables.

Because #2 will take some time to come through the toolchain rolls, and #3 and #4 can be landed asynchronously, we can land this ahead of time. Should make my life easier by preventing constant rebases.

Bug: N/A
Test: Build {libc, libm, libc++, libm, and libnetd} in internal master
with these patches and an experimental compiler with the lld support.
Also, `SANITIZE_TARGET=memtag_globals m` here with in-tree compiler
without lld support.

Change-Id: Ie7882d474d4d776232de3a3d571a82274df14bf0