x86/asm: Add asm macros for static keys/jump labels

Unfortunately, we can only do this if HAVE_JUMP_LABEL.  In
principle, we could do some serious surgery on the core jump
label infrastructure to keep the patch infrastructure available
on x86 on all builds, but that's probably not worth it.

Implementing the macros using a conditional branch as a fallback
seems like a bad idea: we'd have to clobber flags.

This limitation can't cause silent failures -- trying to include
asm/jump_label.h at all on a non-HAVE_JUMP_LABEL kernel will
error out.  The macro's users are responsible for handling this
issue themselves.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/63aa45c4b692e8469e1876d6ccbb5da707972990.1447361906.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>