ELF symbol table field st_other support, 
excluding visibility bits.

Mips specific standalone assembler directive "set at".

This directive changes the general purpose register
that the assembler will use when given the symbolic
register name $at.

This does not include negative testing. That will come
in a future patch.

A side affect of this patch recognizes the different 
GPR register names for temporaries between old abi
and new abi so a test case for that is included.

Contributer: Vladimir Medic


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175686 91177308-0d34-0410-b5e6-96231b3b80d8