gcc: xtensa: add -m[no-]strict-align option

gcc/
	* config/xtensa/xtensa-opts.h: New header.
	* config/xtensa/xtensa.h (STRICT_ALIGNMENT): Redefine as
	xtensa_strict_align.
	* config/xtensa/xtensa.cc (xtensa_option_override): When
	-m[no-]strict-align is not specified in the command line set
	xtensa_strict_align to 0 if the hardware supports both unaligned
	loads and stores or to 1 otherwise.
	* config/xtensa/xtensa.opt (mstrict-align): New option.
	* doc/invoke.texi (Xtensa Options): Document -m[no-]strict-align.