powerpc64: Rewrite strcmp in asm to take advantage of word size

powerpc64: Rewrite strcmp in asm to take advantage of word size

Summary:
Optimize strcmp for powerpc64.
Data is loaded by double words and cmpb intruction is used to find '\0'.

Some performance gain rates between the current and the optimized solution:

String size (bytes)		Gain rate
	<=8			0.59%
	<=16			1.92%
	32			3.02%
	64			5.60%
	128			10.16%
	256			18.05%
	512			30.18%
	1024			42.82%

Submitted by:	alexandre.yamashita_eldorado.org.br,
		leonardo.bianconi_eldorado.org.br
Differential Revision: https://reviews.freebsd.org/D15220