[hwasan] Android: Switch from TLS_SLOT_TSAN(8) to TLS_SLOT_SANITIZER(6)

Summary:
The change is needed to support ELF TLS in Android. See D55581 for the
same change in compiler-rt.

Reviewers: srhines, eugenis

Reviewed By: eugenis

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D55592

llvm-svn: 348983