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.