PR30046, power cmpi leads to unknown architecture

PowerPC ELF always uses bfd_arch_powerpc, so we shouldn't allow the
gas -mpwr, -mpwr2 or -mpwrx options to choose bfd_arch_rs6000.
Given the possible values of ppc_cpu, I think the as_fatal at the end
of ppc_arch will never be reached, so it can be deleted and the code
simplified a little.

	PR 30046
	* config/tc-ppc.c (ppc_arch): Return bfd_arch_powerpc for ELF.
	Delete dead code.

(cherry picked from commit 4d826e13)