ppc: switch handling of 'foreign import wrapper' (FIW) to libffi

Joseph Jezak reported darcs-2.4.4 SIGSEGV in interactive mode in ghc-6.12.3.
So I've concluded ppc also has rotten native adjustor. I don't have hardware
to verify the patch (ticket #3516 should help to test it), but I think it will
help (as similar patch helped for ia64 and ppc64).

mk/config.mk.in

@@ -178,7 +178,7 @@ endif
 # Whether to use libffi for adjustors (foreign import "wrapper") or
 # not.  If we have built-in support (rts/Adjustor.c) then we use that,
 # otherwise we fall back on libffi, which is slightly slower.
-ArchHasAdjustorSupport=$(strip $(if $(findstring $(HostArch_CPP),i386 x86_64 powerpc,),YES,NO))
+ArchHasAdjustorSupport=$(strip $(if $(findstring $(HostArch_CPP),i386 x86_64,),YES,NO))
 ifeq "$(ArchHasAdjustorSupport)" "YES"
 UseLibFFIForAdjustors=NO
 else