MIPS: R6: load/store can process unaligned address

MIPS: R6: load/store can process unaligned address

MIPS release 6 requires the lw/ld/sw/sd can work with
unaligned address, while it can be implemented by
full hardware or trap&emulate.

Since it doesn't have to be fully done by hardware, we add a
pair of options -m(no-)unaligned-access. Kernels may need them.

gcc/ChangeLog:

	* config/mips/mips.h (ISA_HAS_UNALIGNED_ACCESS, STRICT_ALIGNMENT):
		R6 can unaligned access.
	* config/mips/mips.md (movmisalign<mode>): Likewise.
	* config/mips/mips.opt: add -m(no-)unaligned-access
	* doc/invoke.texi: Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/mips/mips.exp: add unaligned-access
	* gcc.target/mips/unaligned-2.c: New test.
	* gcc.target/mips/unaligned-3.c: New test.