eliminate the PPC backend's implementation of EmitExternalGlobal

and use PersonalityPrefix/Suffix to achieve the same effect (like
the x86 backend).

This changes the code generated for ppc static mode, but guess what,
we were generating this before:

	.byte	0x9B                                        ; Personality (indirect pcrel sdata4)
	.long	___gxx_personality_v0-.                     ; Personality

which is not correct! (it is not an 'indirect' reference).
 

llvm-svn: 81965