Switch Android from TLS_SLOT_TSAN(8) to TLS_SLOT_SANITIZER(6)

Summary:
The TLS_SLOT_TSAN slot is available starting in N, but its location (8)
is incompatible with the proposed solution for implementing ELF TLS on
Android (i.e. bump ARM/AArch64 alignment to reserve an 8-word TCB).

Instead, starting in Q, Bionic replaced TLS_SLOT_DLERROR(6) with
TLS_SLOT_SANITIZER(6). Switch compiler-rt to the new slot.

Reviewers: eugenis, srhines, enh

Reviewed By: eugenis

Subscribers: ruiu, srhines, kubamracek, javed.absar, kristof.beyls, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D55581

llvm-svn: 348984