[JITLink][Mips] Add ELF backend (#224578)

Add support for O32, N32, and N64 relocatable ELF objects in both byte
orders. Handle O32 implicit addends and HI/LO pairing, N32 relocation
groups, and N64 packed relocations. Preserve the MIPS64 ISA triple for
N32 while recording its four-byte pointer ABI in LinkGraph.

Implement absolute, PC-relative, GP/GOT, and compound fixups, GOT-backed
$t9 call stubs, and EH-frame support. Use MIPS-specific GOT keys for
target, addend, and exact/page entries, and collect GP-relative content
into one ordered region. Support standard MIPS ISAs, including release-6
stubs.

Integrate GD/LD TLS with ELFNixPlatform and add an assembly resolver
that reads the thread pointer with rdhwr before calling the existing C
implementation. Honor explicit MIPS relocation-model requests while
retaining the static default. JITLink remains opt-in and RuntimeDyld
remains the default.

Add focused relocation, ABI, GOT/stub, EH-frame, and TLS tests.

Assisted-by: chatgpt-5.6-sol
NOKEYCHECK=True
GitOrigin-RevId: 2820795b027eb68e3c3a0f1ad1ddd36aaeb8730a