scudo: Update TLS_SLOT_SANITIZER value.

Android now allocates only 8 fixed TLS slots. Somehow we were getting away
with using a non-existent slot until now, but in some cases the TLS slots
were being placed at the end of a page, which led to a segfault at startup.

Differential Revision: https://reviews.llvm.org/D69191

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@375276 91177308-0d34-0410-b5e6-96231b3b80d8