[mips] Do not include offset into `%got` expression for global symbols

Now pseudo instruction `la $6, symbol+8($6)` is expanding into the following
chain of commands:
```
lw    $1, %got(symbol+8)($gp)
addiu $1, $1, 8
addu  $6, $1, $6
```

This is incorrect. When a linker handles the `R_MIPS_GOT16` relocation,
it does not expect to get any addend and breaks on assertion. Otherwise
it has to create new GOT entry for each unique "sym + offset" pair.
Offset for a global symbol should be added to result of loading GOT
entry by a separate `add` command.

The patch fixes the problem by stripping off an offset from the expression
passed to the `%got`. That's interesting that even current code inserts
a separate `add` command.

Differential Revision: https://reviews.llvm.org/D66552

llvm-svn: 369755