[Powerpc] Tune/optimize powerpc{32,64}/power7/memchr.S.

Assorted tweaking, twisting and tuning to squeeze a few additional cycles
out of the memchr code.   Changes include bypassing the shift pairs
(sld,srd) when they are not required, and unrolling the small_loop that
handles short and trailing strings.

Per scrollpipe data measuring aligned strings for 64-bit, these changes
save between five and eight cycles (9-13% overall) for short strings (<32),
Longer aligned strings see slight improvement of 1-3% due to bypassing the
shifts and the instruction rearranging.