MIPS/GAS: Add HI16/LO16 pairing for REL TLS relocs

MIPS/GAS: Add HI16/LO16 pairing for REL TLS relocs

Complementing commit 7ea90d9316d3 ("MIPS: Fix linker for REL TLS
HI16/LO16 relocs") also add pairing for HI16/LO16 REL TLS relocations
in GAS, which is where it needs to be done in the first place and
which is required for later linker operation on the objects produced.

Pairing also corrects in-place addend installation for the high-part
relocations, which used not to happen in the absence of this fix for
ones not already followed by the corresponding low-part relocation.

Add test cases to verify relocation ordering and addend installation.