x86: change asm constraint.

Our integration efforts broke a build with this function being used
with i386. Reason is "g" can put the operand in an imm32, which according
to The Book (tm), is invalid as the second operand.

This is actually a bug
in x86_64 too, since the x86_64 instruction set reference does not list
it as valid.

We probably didn't trigger this before due to the ammount of
registers available for 64-bit platforms. But that's just my guess.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>