[mips] Improve error/warning messages and testing for the .cpload assembler directive.

Summary:
Improved warning message when using .cpload inside a reorder section and added an error message for using .cpload with Mips16 enabled.
Modified the tests to fit with the changes mentioned above, added a test-case for the N32 ABI in cpload.s and did some reformatting to make the tests easier to read.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5465