[x86/retpoline] Make the external thunk names exactly match the names

that happened to end up in GCC.

This is really unfortunate, as the names don't have much rhyme or reason
to them. Originally in the discussions it seemed fine to rely on aliases
to map different names to whatever external thunk code developers wished
to use but there are practical problems with that in the kernel it turns
out. And since we're discovering this practical problems late and since
GCC has already shipped a release with one set of names, we are forced,
yet again, to blindly match what is there.

Somewhat rushing this patch out for the Linux kernel folks to test and
so we can get it patched into our releases.

Differential Revision: https://reviews.llvm.org/D42998

llvm-svn: 324449