[mips] Replace MipsABIEnum with a MipsABIInfo class.

Summary:
No functional change yet, it's just an object replacement for an enum.
It will allow us to gather ABI information in a single place so that we can
start testing for properties of the ABI's instead of the ABI itself.

For example we will eventually be able to use:
  ABI.MinStackAlignmentInBytes()
instead of:
  (isABI_N32() || isABI_N64()) ? 16 : 8
which is clearer and more maintainable.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3341