[PowerPC] Re-enable VSX test line for fma.ll with -mcpu=pwr7

The VSX testing variant in test/CodeGen/PowerPC/fma.ll had to be
disabled because of unexpected behavior on many of the builders.  I
tracked this down to a situation that occurs when the VSX attribute is
enabled for a target that disables the MI early scheduling pass.  This
patch adds -mcpu=pwr7 to make this predictable.  The other issue will
be addressed separately.