rs6000: Make P10_FUSION honour tuning setting

rs6000: Make P10_FUSION honour tuning setting

We noticed this issue when Segher reviewed the patch for
PR104024.  When there is no explicit setting for option
-mpower10-fusion, we enable OPTION_MASK_P10_FUSION for
TARGET_POWER10.  But it's not right, it should honour
tuning setting instead.

This patch is to fix it accordingly, it's bootstrapped
, and regtested on powerpc64-linux-gnu P8 and
powerpc64le-linux-gnu P9.

But on powerpc64le-linux-gnu P10 it had one regression
failure against the test case gcc.target/powerpc/pr105586.c.
I looked into it and confirmed that a latent bug was
exposed and filed one separated bug PR108273 instead.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Make
	OPTION_MASK_P10_FUSION implicit setting honour Power10 tuning setting.
	* config/rs6000/rs6000-cpus.def (ISA_3_1_MASKS_SERVER): Remove
	OPTION_MASK_P10_FUSION.