Remove TLS_SLOT_TSAN(8)

As of the switch to clang-r346389c, it has been replaced with
TLS_SLOT_SANITIZER(6). lld reserves 8 words beyond the TP on arm/arm64, so
Bionic can't use anything beyond 7.

The DTV and bionic_tls slots on x86 haven't been part of a release yet,
and they should be strictly internal to Bionic anyway, so shift them down.

Bug: http://b/78026329
Test: bionic unit tests
Change-Id: Ifb3b2d8d85efe1417ee9a10b657b665ec6f2fd3d