[mips] Invert the abicalls feature bit to be noabicalls so that it's possible...

[mips] Invert the abicalls feature bit to be noabicalls so that it's possible for -mno-abicalls to take effect.

Also added the testcase that should have been in r215194.

This behaviour has surprised me a few times now. The problem is that the
generated MipsSubtarget::ParseSubtargetFeatures() contains code like this:

   if ((Bits & Mips::FeatureABICalls) != 0) IsABICalls = true;

so '-abicalls' means 'leave it at the default' and '+abicalls' means 'set it to
true'. In this case, (and the similar -modd-spreg case) I'd like the code to be

  IsABICalls = (Bits & Mips::FeatureABICalls) != 0;

or possibly:

   if ((Bits & Mips::FeatureABICalls) != 0)
     IsABICalls = true;
   else
     IsABICalls = false;

and preferably arrange for 'Bits & Mips::FeatureABICalls' to be true by default
(on some triples).

llvm-svn: 215211