[mips] Add support to '.set mips64'.

[mips] Add support to '.set mips64'.

The '.set mips64' directive enables the feature Mips:FeatureMips64
from assembly. Note that it doesn't modify the ELF header as opposed
to the use of -mips64 from the command-line. The reason for this
is that we want to be as compatible as possible with existing assemblers
like GAS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204817 91177308-0d34-0410-b5e6-96231b3b80d8