gn build: Pass -fno-emulated-tls when targeting Android.

Matches the CMake build:
https://github.com/llvm/llvm-project/blob/93c1a5f3ddd41e0ec09f38ab0045bd5e92199fd5/compiler-rt/CMakeLists.txt#L343
(we always use API level 29).

Differential Revision: https://reviews.llvm.org/D146341