rs6000: Consider explicitly set options in target option parsing [PR115713]

rs6000: Consider explicitly set options in target option parsing [PR115713]

In rs6000_inner_target_options, when enabling VSX we enable
altivec and disable -mavoid-indexed-addresses implicitly,
but it doesn't consider the case that the options altivec
and avoid-indexed-addresses can be explicitly disabled.  As
the test case in PR115713#c1 shows, with target attribute
"no-altivec,vsx", it results in that VSX unexpectedly set
altivec flag and there isn't an expected error.

This patch is to avoid the automatic enablement when they
are explicitly specified.  With this change, an existing
test case ppc-target-4.c also requires an adjustment by
specifying explicit altivec in target attribute (since it
requires altivec feature and command line is specifying
no-altivec).

	PR target/115713

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_inner_target_options): Avoid to
	enable altivec or disable avoid-indexed-addresses automatically
	when they get specified explicitly.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr115713-1.c: New test.
	* gcc.target/powerpc/ppc-target-4.c: Adjust by specifying altivec
	in target attribute.