MIPS: Unify sc beqz definition

We currently duplicate the definition of __scbeqz in asm/atomic.h &
asm/cmpxchg.h. Move it to asm/llsc.h & rename it to __SC_BEQZ to fit
better with the existing __SC macro provided there.

We include a tab in the string in order to avoid the need for users to
indent code any further to include whitespace of their own after the
instruction mnemonic.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org