Reserve TLS_SLOT_SANITIZER with value 6.

This slot will replace TLS_SLOT_TSAN, which needs to be removed for ELF
TLS compatibility. The platform has both slots reserved until compiler-rt
can be updated. See:
 - https://reviews.llvm.org/D53906
 - discussion on AOSP change Idf25d2ee457a5d26d0bdd6281cee72c345c8b755

Bug: http://b/78026329
Test: m
Change-Id: Ifd5aeb5711dc88266e7350cb022146dd87cfd7be