um, x86-64: Fix UML build after adding CFI annotations to lib/rwsem_64.S

arch/um/Kconfig.x86 has X86_32 but not X86_64 - that's resulting in
asm/dwarf2.h producing the 32-bit (pushl_cfi & Co) macros instead of
the 64-bit ones.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Jeff Dike <jdike@addtoit.com>
LKML-Reference: <4D6CE3400200007800034498@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>