[mips] Allow constant expressions in second argument of .cpsetup.

Summary:
Also tightened up the test and made a trivial fix to prevent double-newline
after emitting .cpsetup directives.

Reviewers: vkalintiris

Subscribers: seanbruno, emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D12956

llvm-svn: 248143