[mips] [IAS] Implement the .asciiz directive.

Summary:
This directive is exactly the same as .asciz, except it's only used by MIPS.
It is used to store null terminated strings in object files.

Reviewers: rafael, dsanders, echristo

Reviewed By: dsanders, echristo

Subscribers: echristo, llvm-commits

Differential Revision: http://reviews.llvm.org/D7530

llvm-svn: 235382