[CPU-Dispatch] Make pentium_iii_no_xmm_regs and pentium_iii alias.

I discovered that in ICC (where this list comes from), that the two
pentium_iii versions were actually identical despite the two different
names (despite them implying a difference). Because of this, they ended
up having identical manglings, which obviously caused problems when used
together.

This patch makes pentium_iii_no_xmm_regs an alias for pentium_iii so
that it can still be used, but has the same meaning as ICC. However, we
still prohibit using the two together which is different (albeit better)
behavior.

Change-Id: I4f3c9a47e48490c81525c8a3d23ed4201921b288
llvm-svn: 352054