rs6000: Set param_vect_partial_vector_usage as 1 for P10

rs6000: Set param_vect_partial_vector_usage as 1 for P10

Due to the unexpected performance on Power9 of those vector
with length instructions, we didn't enable vectorization with
partial vectors before.

Some recent testings show that they perform expectedly on
Power10 now.  This patch is to set
param_vect_partial_vector_usage as 1 for P10 or later.

The performance evaluation on the whole SPEC2017 with option
set power10/Ofast/unroll shows it can speed up 525.x264_r by
10.80% and 554.roms_r by 1.94%.

Bootstrapped/regtested on powerpc64le-linux-gnu P10.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Set param_vect_partial_vector_usage as 1 for Power10 and up
	by default.