[mips] Handle variables with an explicit section and interactions with .sdata, .sbss

If a variable has an explicit section such as .sdata or .sbss, it is placed
in that section and accessed in a gp relative manner. This overrides the global
-G setting.

Otherwise if a variable has a explicit section attached to it, such as '.rodata'
or '.mysection', it is not placed in the small data section. This also overrides
the global -G setting.

Reviewers: atanasyan, nitesh.jain

Differential Revision: https://reviews.llvm.org/D36616

llvm-svn: 311001