rs6000: Fix .machine cpu selection w/ altivec [PR97367]

There are various non-IBM CPUs with altivec, so we cannot use that
flag to determine which .machine cpu to use, so ignore it.
Emit an additional ".machine altivec" if Altivec is enabled so
that the assembler doesn't require an explicit -maltivec option
to assemble any Altivec instructions for those targets where
the ".machine cpu" is insufficient to enable Altivec.  For example,
-mcpu=G5 emits a ".machine power4".

2024-07-18  René Rebe  <rene@exactcode.de>
	    Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/97367
	* config/rs6000/rs6000.cc (rs6000_machine_from_flags): Do not consider
	OPTION_MASK_ALTIVEC.
	(emit_asm_machine): For Altivec compiles, emit a ".machine altivec".

gcc/testsuite/
	PR target/97367
	* gcc.target/powerpc/pr97367.c: New test.

Signed-off-by: René Rebe <rene@exactcode.de>