rs6000: Parameterize some const values for density test

rs6000: Parameterize some const values for density test

This patch follows the discussion here[1], where Segher suggested
parameterizing those exact magic constants for density heuristics,
to make it easier to tweak if need.

The change here should be "No Functional Change".  But I verified
it with SPEC2017 at option sets O2-vect and Ofast-unroll on Power8,
the result is neutral as expected.

[1]https://gcc.gnu.org/pipermail/gcc-patches/2021-September/579121.html

gcc/ChangeLog:

	* config/rs6000/rs6000.opt (rs6000-density-pct-threshold,
	rs6000-density-size-threshold, rs6000-density-penalty,
	rs6000-density-load-pct-threshold,
	rs6000-density-load-num-threshold): New parameter.
	* config/rs6000/rs6000.c (rs6000_density_test): Adjust with
	corresponding parameters.