rs6000: add option -mblock-ops-unaligned-vsx

rs6000: add option -mblock-ops-unaligned-vsx

This option is mostly being added to provide -mno-block-ops-unaligned-vsx.
The default is set the same as -mefficient-unaligned-vsx. This option will
control the use of unaligned VSX loads/stores in the inline expansion
of memcpy() and memmove(). The use case for this would be if you're
compiling code that is doing a memcpy to memory mapped device memory
that is cache-inhibited. On some powerpc processors this requires the
unaligned vsx ops to be emulated by the kernel which is very slow.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Set the default value for -mblock-ops-unaligned-vsx.
	* config/rs6000/rs6000.opt: Add -mblock-ops-unaligned-vsx.
	* doc/invoke.texi: Document -mblock-ops-unaligned-vsx.