[mips] The naming convention for private labels is ABI dependant.

Summary:
For N32/N64, private labels begin with '.L' but for O32 they begin with '$'.

MCAsmInfo now has an initializer function which can be used to provide information from the TargetMachine to control the assembly syntax.

Reviewers: vkalintiris

Reviewed By: vkalintiris

Subscribers: jfb, sandeep, llvm-commits, rafael

Differential Revision: http://reviews.llvm.org/D9821

llvm-svn: 237789