elf: fix MIPS64 little-endian relocation parsing

[messense]

Summary

Fixes #274 — MIPS64 LE ELF binaries fail to parse with an out-of-bounds error because r_sym and r_type return garbage values from the non-standard MIPS64 relocation layout.

Background

MIPS64 ELF uses a non-standard relocation info (r_info) layout different from standard ELF64:

Standard ELF64	MIPS64
r_sym (32 bits) | r_type (32 bits)	r_sym (32 bits) | r_ssym (8 bits) | r_type3 (8 bits) | r_type2 (8 bits) | r_type (8 bits)

On little-endian MIPS64, when this struct is read as a single u64, the byte order causes the fields to be scrambled. For example, an r_info with r_sym=0 and r_type=R_MIPS_REL32 gets read as 0x0312000000000000, causing:

• r_sym to return 51,511,296 (instead of 0)
• r_type to return 0 (instead of 3)

The bogus r_sym then causes the dynamic symbol table parsing to try to read millions of symbols, failing with an out-of-bounds error.

Approach

Based on the maintainer's feedback from #382 — pass down whether it's a MIPS binary at runtime rather than using #[cfg] compile-time conditionals:

1. reloc64::mips64el_r_info() — Converts MIPS64 LE r_info to standard ELF64 format using the same byte transformation as LLVM and the object crate

2. Reloc::fixup_mips64el() — Applies the transformation after parsing a relocation entry

3. RelocSection / RelocIterator — Carry an is_mips64el flag and conditionally apply the fixup during get() and iteration

4. Elf::parse_with_opts() — Detects MIPS64 LE from the ELF header (e_machine == EM_MIPS && 64-bit && little-endian) and passes it through via RelocSection::parse_inner()

Backward Compatibility

• RelocSection::parse() signature is unchanged (defaults is_mips64el = false)
• RelocCtx type alias is unchanged
• r_sym() / r_type() / r_info() functions are unchanged
• All existing tests pass

Tests

Added 5 new tests:

• test_mips64el_r_info — validates the byte transformation with real data from MIPS64 parse error: "type is too big (1236271128) for 137416" #274
• test_mips64el_r_info_with_sym — validates transformation when r_sym is non-zero
• test_mips64el_r_info_zero — edge case: all-zero r_info
• test_standard_r_sym_r_type_unchanged — ensures no regression for non-MIPS
• test_mips64el_reloc_section_parse — integration test through the full RelocSection pipeline, verifying both broken (without fix) and correct (with fix) behavior

References

• Issue: MIPS64 parse error: "type is too big (1236271128) for 137416" #274
• Previous PR: elf.reloc: fix mips64 parse error #382 (closed with feedback to avoid #[cfg])
• MIPS64 ELF ABI documentation
• LLVM implementation
• object crate implementation
• Binutils MIPS64 relocation structs