[tablegen] Add CustomCallingConv and use it to tablegen-erate the outermost parts of the Mips O32 implementation

Summary:
CustomCallingConv is simply a CallingConv that tablegen should not generate the
implementation for. It allows regular CallingConv's to delegate to these custom
functions. This is (currently) necessary for Mips and we cannot use CCCustom
without having to adapt to the different API that CCCustom uses.

This brings us a bit closer to being able to remove
MipsCC::analyzeCallOperands and MipsCC::analyzeFormalArguments in favour of
the common implementation.

No functional change to the targets.

Depends on D3341

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: vmedic, llvm-commits

Differential Revision: http://reviews.llvm.org/D5965