Update -maltivec and -mvsx options to only enable their oldest instructions.

Update -maltivec and -mvsx options to only enable their oldest instructions.

Currently, the -maltivec and -mvsx GAS options enable *all* of the altivec
and vsx instructions respecitively that have ever been added.  This is in
constract to GCC's -maltivec and -mvsx options, which only enable the oldest
(ie, first) set of altivec and vsx instructions.  This patch changes GAS to
mimic GCC's behaviour with respect to -maltivec and -mvsx and it solves a
problem with trying to assemble the lxvx instruction which is different
between POWER8 and POWER9.

opcodes/
	* ppc-dis.c (ppc_opts) <altivec>: Do not use PPC_OPCODE_ALTIVEC2;
	<vsx>: Do not use PPC_OPCODE_VSX3;

gas/
	* testsuite/gas/ppc/altivec2.d (as): Use the -mpower8 option.
	(objdump): Use the -Mpower8 option.