[MIPS] Implement support for -mstack-alignment.

Summary:
This is modeled on the implementation for x86 which stores the command line
option in a 'StackAlignOverride' field in MipsSubtarget and then uses this
to compute a 'stackAlignment' value in
MipsSubtarget::initializeSubtargetDependencies.

The stackAlignment() method in MipsSubTarget is renamed to getStackAlignment()
and returns the computed 'stackAlignment'.

Reviewers: sdardis

Reviewed By: sdardis

Subscribers: llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D35874

llvm-svn: 310891