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

[pcc]

Summary

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

llvm/utils/gn/build/toolchain/target_flags.gni

 import("//llvm/triples.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 # Flags in this file are passed both to the compiler that's building
 # compiler-rt at build time (via normal gn cflags/ldflags), as well as to the
 # compiler building compiler-rt test programs at test time (via
 # COMPILER_RT_TEST_COMPILER_CFLAGS).
 
 target_flags = []
 target_ldflags = []
 
 if (current_os == "android") {
   target_flags += [
     "--target=$llvm_current_triple",
     "--sysroot=$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64/sysroot",
     "--gcc-toolchain=$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64",
+    "-fno-emulated-tls",
   ]
   target_ldflags += [
     "-static-libstdc++",
     "--unwindlib=none",
   ]
   if (current_cpu == "arm") {
     target_flags += [ "-march=armv7-a" ]
   }