Support mips / mipsle / mips64 / mips64le

[egonelbre]

Part of #1.

Map the ELF machine types in internal/objfile. Note: golang.org/x/arch has no MIPS disassembler, so this needs a third-party decoder or writing one — lowest priority of the missing architectures.

[egonelbre]

Investigated available decoders — no viable option today:

• golang.org/x/arch has no mips package; upstream tracking issue cmd/objdump: implement mips disassembler golang/go#19158 ("cmd/objdump: implement mips disassembler") has been open since 2017, milestone Unplanned. Even go tool objdump cannot disassemble GOARCH=mips* binaries.
• mewmew/mips (and pippi, which wraps it): MIPS I only — no MIPS32r2/MIPS64/FPU, so it can't cover what Go's compiler emits. Inactive.
• unixpickle/mips32: educational, ~40 instructions, no license, unmaintained.
• Capstone bindings (bnagy/gapstone): full coverage but cgo, which breaks ixdiff's pure-Go install story.

Writing a decoder in-tree would mean maintaining an x/arch-quality package (~200–300 encodings for MIPS32r1/r2 + MIPS64 + FPU, both endiannesses, plus a conformance corpus) — the wrong home for it. The right path is contributing golang.org/x/arch/mips/mipsasm upstream (golang/go#19158); once a decoder exists, ixdiff's plumbing (objfile mapping, Decode case, LUI+ADDIU pairwise masking like the arm64 ADRP+ADD rules) is a small add.

Parking this until an upstream decoder exists.