scudo: Only use the Android reserved TLS slot when building libc's copy of the allocator.

When we're not building libc's allocator, just use a regular TLS variable. This
lets the unit tests pass on Android devices whose libc uses Scudo. Otherwise
libc's copy of Scudo and the unit tests' copy will both try to use the same
TLS slot, in likely incompatible ways.

This requires using ELF TLS, so start passing -fno-emulated-tls when building
the library and the unit tests on Android.

Differential Revision: https://reviews.llvm.org/D70472