[RS6000] PowerPC -mcpu=native support

[RS6000] PowerPC -mcpu=native support

The -mcpu=native support has bit-rotted a little, in particular the
fallback when the native cpu couldn't be determined.  This patch fixes
the bit-rot and reorganizes ASM_CPU_SPEC so that it should be a little
easier to keep the -mcpu=native data up to date.

The patch also changes the fix for PR63177 (-mpower9-vector being
passed by the user when the default is -mpower8) to also apply when
-mcpu=powerpc64le and -mcpu=native is given.  I'll note that the hack
for PR63177 should probably be extended to lots of other options, if
we're going to continue supporting all those sub-architecture options
(-mpower9-vector, -mpower8-vector, -mcrypto, -mdirect-move, -mhtm,
-mvsx and others) in the positive sense.  I think those should have
only been supported in their -mno- variants..

	* config/rs6000/aix71.h (ASM_SPEC): Don't select default -maix64
	cpu here.
	(ASM_CPU_SPEC): Do so here.  Rewrite using if .. else if .. specs
	form.  Error on missing -mcpu case.
	* config/rs6000/driver-rs6000.c (asm_names <_AIX>): Update NULL case.
	(asm_names <!_AIX>): Add missing cpus.  Update NULL case.  Apply
	PR63177 fix for -mcpu=power8 and -mcpu=powerpc64le.
	* config/rs6000/rs6000.h (ASM_CPU_SPEC): Rewrite using if ..
	else if .. specs form.  Error on missing -mcpu case.  Don't output
	duplicate -maltivec.  Apply PR63177 fix for -mcpu=powerpc64le.

From-SVN: r266502