fix(aot): reserve x18 register on macOS ARM64

[teamchong]

Problem

Apple reserves CPU register x18 for TLS on ARM64. When generating AOT code for aarch64 on macOS, LLVM may use x18, causing crashes when the AOT code runs on macOS ARM64 (M1/M2/M3).

Solution

This patch makes three changes to properly handle macOS ARM64:

1. Detect darwin/macho ABI - When explicit ABI is specified as "darwin" or "macho", use -apple- vendor string
2. Detect darwin/apple in default triple - When auto-detecting platform, check for "darwin" or "apple" in the triple
3. Add +reserve-x18 feature - Tell LLVM to reserve x18 register when targeting aarch64 on macOS hosts

#if (defined(__APPLE__) || defined(__MACH__)) && defined(BUILD_TARGET_AARCH64)
    if (is_aarch64) {
        // Append +reserve-x18 to features
    }
#endif

Testing

• AOT compilation and execution on macOS ARM64 (M1/M2/M3) works correctly
• No crashes from x18 register conflicts

Related

• AOT compiler wamrc doesn't support compiling to arm64 bit #3164
• Using wamrc for target aarch64 throws an error #1944