Add ARM EABI function __aeabi_read_tp

By default, Clang uses this arm32 function to read the thread pointer,
either for ELF TLS or via __builtin_thread_pointer(). It's probably better
to inline the cp15 access using -mtp=cp15, but that's not the default yet.

See https://reviews.llvm.org/D34878?id=114573.

Bug: http://b/78026329
Test: bionic unit tests
Change-Id: I93b8926075f0b2cea8df9ef518d54f2820a8ff5b