MIPS: Add -m(no-)strict-align option

MIPS: Add -m(no-)strict-align option

We support options -m(no-)unaligned-access 2 years ago, while
currently most of other ports prefer -m(no-)strict-align.
Let's support -m(no-)strict-align, and keep -m(no-)unaligned-access
as alias.

gcc
	* config/mips/mips.opt: Support -mstrict-align, and use
	TARGET_STRICT_ALIGN as the flag; keep -m(no-)unaligned-access
	as alias.
	* config/mips/mips.h: Use TARGET_STRICT_ALIGN.
	* config/mips/mips.opt.urls: Regenerate.
	* doc/invoke.texi: Document -m(no-)strict-algin for MIPSr6.