[RS6000] -mno-minimal-toc vs. power10 pcrelative

[RS6000] -mno-minimal-toc vs. power10 pcrelative

We've had this hack in the libgcc config to build libgcc with
-mcmodel=small for powerpc64 for a long time.  It wouldn't be a bad
thing if someone who knows the multilib machinery well could arrange
for -mcmodel=small to be passed just for ppc64 when building for
earlier than power10.  But for now, make -mno-minimal-toc do nothing
when pcrel.  Which will do the right thing for any project that has
copied libgcc's trick.

We want this if configuring using --with-cpu=power10 to build a
power10 pcrel libgcc.  --mcmodel=small turns off pcrel.

gcc/
	* config/rs6000/linux64.h (SUBSUBTARGET_OVERRIDE_OPTIONS): Don't
	set -mcmodel=small for -mno-minimal-toc when pcrel.
libgcc/
	* config/rs6000/t-linux: Document purpose of -mno-minimal-toc.