rs6000: Escalate warning to error for VSX with explicit no-altivec etc.

rs6000: Escalate warning to error for VSX with explicit no-altivec etc.

As the discussion in PR115688, for now when users specify
-mvsx and -mno-altivec explicitly, compiler emits warning
rather than error, but considering both options are given
explicitly, emitting hard error should be better.

So this patch is to escalate some related warning to error
when both are incompatible.

	PR target/115713

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Emit error
	messages when explicit VSX encounters explicit soft-float, no-altivec
	or avoid-indexed-addresses.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/warn-1.c: Move to ...
	* gcc.target/powerpc/error-1.c: ... here.  Adjust dg-warning with
	dg-error and remove ineffective scan.