x86/relocs: Handle R_X86_64_REX_GOTPCRELX relocations

Clang may produce R_X86_64_REX_GOTPCRELX relocations when redefining the
stack protector location.  Treat them as another type of PC-relative
relocation.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250123190747.745588-6-brgerst@gmail.com