[AddressSanitizer] Don't use weak linkage for __{start,stop}_asan_globals

It should not be necessary to use weak linkage for these. Doing so
implies interposablity and thus PIC generates indirections and
dynamic relocations, which are unnecessary and suboptimal. Aside
from this, ASan instrumentation never introduces GOT indirection
relocations where there were none before--only new absolute relocs
in RELRO sections for metadata, which are less problematic for
special linkage situations that take pains to avoid GOT generation.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D80605