ARM: Use the PICADD opcode calculated.

We were figuring out whether to use tPICADD or PICADD, then just using
tPICADD unconditionally anyway. Oops.

A testcase from someone familiar enough with ELF to produce one would
be appreciated. The existing PIC testcase correctly verifies the .s
generated, but that doesn't catch this bug, which only showed up in
direct-to-object mode.

http://llvm.org/bugs/show_bug.cgi?id=17180