PowerPC: ignore sticky options for .machine

PowerPC gas and objdump for a long time have allowed certain -m/-M
options that extend a base cpu with extra functional units to be
specified before the base cpu.  For example, "-maltivec -mpower4" is
the same as "-mpower4 -maltivec".  See
https://sourceware.org/pipermail/binutils/2008-January/054935.html

It doesn't make as much sense that .machine keep any of these
"sticky" flags when handling a new base cpu.  See gcc PR101393.  I
think that instead .machine ought to override the command line.
That's what this patch does.  It is still possible to extend cpu
functionality with .machine.  For example the following can be
assembled when selecting a basic -mppc on the command line:
	.machine power5
	.machine altivec
	frin 1,2
	lvsr 3,4,5
Here, ".machine altivec" extends the ".machine power5" so that both
the power5 "frin" instruction and the altivec "lvsr" instruction are
enabled.  Swapping the two ".machine" directives would result in
failure to assemble "lvsr".

This change will expose some assembly errors, such as the one in
glibc/sysdeps/powerpc/powerpc64/tst-ucontext-ppc64-vscr.c, a file
compiled with -maltivec but containing
  asm volatile (".machine push;\n"
		".machine \"power5\";\n"
		"vspltisb %0,0;\n"
		"vspltisb %1,-1;\n"
		"vpkuwus %0,%0,%1;\n"
		"mfvscr %0;\n"
		"stvx %0,0,%2;\n"
		".machine pop;"
		: "=v" (v0), "=v" (v1)
		: "r" (vscr_ptr)
		: "memory");
It's just wrong to choose power5 for a bunch of altivec instructions
and in fact all of those .machine directives are unnecessary.

	* config/tc-ppc.c (ppc_machine): Don't use command line
	sticky options.

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