rs6000: Fix -mpower9-vector -mno-altivec ICE (PR87560)

rs6000: Fix -mpower9-vector -mno-altivec ICE (PR87560)

PR87560 reports an ICE when a test case is compiled with -mpower9-vector
and -mno-altivec.  This patch terminates compilation with an error when
this combination (and other unreasonable ones) are requested.

Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
regressions.  Reported error is now:

f951: Error: '-mno-altivec' turns off '-mpower9-vector'

2020-03-02  Bill Schmidt  <wschmidt@linux.ibm.com>

	PR target/87560
	* rs6000-cpus.def (OTHER_ALTIVEC_MASKS): New #define.
	* rs6000.c (rs6000_disable_incompatible_switches): Add table entry
	for OPTION_MASK_ALTIVEC.