test/rs6000: Add Power9 and up as vect_len target

test/rs6000: Add Power9 and up as vect_len target

Power9 supports vector with length in bytes load/store, this patch
is to teach check_effective_target_vect_len_load_store to take it
and its laters as effective vector with length targets.

Also supplement the documents for has_arch_pwr*.

Bootstrapped/regtested on powerpc64le-linux-gnu P8, also on
powerpc64le-linux-gnu P9 with explicit usage setting.

gcc/ChangeLog:

	* doc/sourcebuild.texi (has_arch_pwr5, has_arch_pwr6, has_arch_pwr7,
	has_arch_pwr8, has_arch_pwr9): Document.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp
	(check_effective_target_vect_len_load_store): Call check function
	check_effective_target_has_arch_pwr9.