[AArch64] Fix D19098 fallout.

The intrinsic is now called llvm.thread.pointer, not
llvm.aarch64.thread.pointer.  Also, the code handling it in CGBuiltin.cpp
is dead - it's already covered by GCCBuiltin.  Remove it.

Differential Revision: http://reviews.llvm.org/D19099

llvm-svn: 266817