Add ARM64EC support

[messense]

Implements generation of ARM64EC (Emulation Compatible) Windows import libraries, verified byte-identical to llvm-dlltool output.

Changes:

• Add ARM64EC variant to MachineType (IMAGE_FILE_MACHINE_ARM64EC = 0xA641)
• MSVC flavor: use native ARM64 for descriptor objects, ARM64EC for short imports; mangle code import names with # prefix (or $$h insertion for C++ names) and emit them via IMPORT_NAME_EXPORTAS
• Archive writer: write second linker member (LE, name-sorted with u16 member indices) and /<ECSYMBOLS>/ member required for COFF/EC archives
• Split symbols across regular and EC tables: descriptors go in regular (duplicated to EC), exports go in EC only
• GNU flavor: map ARM64EC to Aarch64 with ARM64 jmp stubs/relocations
• Update object crate to 0.39.1 (replace deprecated U16Bytes/U32Bytes aliases with U16/U32)
• Add lightweight MSVC mangled-name parser (~130 lines) to find the $$h insertion point without depending on a full demangler

[Copilot]

Pull request overview

Adds ARM64EC support across MSVC and GNU import library generation, including archive-format updates needed for ARM64EC COFF archives.

Changes:

• Introduces MachineType::ARM64EC and maps it appropriately in MSVC/GNU generation paths.
• MSVC: adds ARM64EC name mangling + IMPORT_NAME_EXPORTAS, and splits symbols between regular and EC symbol tables.
• Archive writer: adds /<ECSYMBOLS>/ support and writes a second linker member when generating EC archives; updates object dependency to 0.39.1.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/lib.rs	Adds ARM64EC machine type and a native-machine mapping used by MSVC descriptor objects.
src/msvc.rs	Implements ARM64EC import name mangling, EXPORTAS handling, descriptor-vs-EC symbol table splitting, and adds unit tests for mangling logic.
src/gnu.rs	Maps ARM64EC to AArch64 and reuses ARM64 stubs/relocations; minor error-construction refactors.
src/ar/write.rs	Adds a second linker member for EC archives and emits the /<ECSYMBOLS>/ member; supports dual symbol tables.
src/ar/mod.rs	Defines EC_SYMBOL_TABLE_ID constant for the new member.
src/def/parser.rs	Minor refactor: derive-based Default for TokenKind and small control-flow cleanup.
tests/test_import_lib.rs	Adds ARM64EC tests for MSVC and GNU flavors (light structural assertion for MSVC).
Cargo.toml	Bumps object crate to 0.39.1.

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