x86: don't save ebx in putuser_32.S.

Clobber it in the inline asm macros, and let the compiler do this for us.

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>