Fix ARM64EC builds by excluding ARM64EC from x64(_M_X64) detection on Windows

[navvyswethgraphics]

Issue description

On Windows builds using MSVC, ARM64EC defines _M_X64 for source‑level compatibility.
As a result, ARM64EC builds were incorrectly classified as "OJPH_ARCH_X86_64", enabling x86-64 specific code paths and build flags. This leads to compilation failures due to unsupported intrinsics and incorrect ABI assumptions.

Build failure evidence (before fix)

1>------ Rebuild All started: Project: ZERO_CHECK, Configuration: Debug ARM64EC ------
1>1>Checking Build System
2>------ Rebuild All started: Project: openjph, Configuration: Debug ARM64EC ------
2>Compiling...
2>Generating Code...
2>ojph_mem_c.c
2>cl : command line warning D9002: ignoring unknown option '/arch:AVX'
2>ojph_codestream_avx.cpp
2>ojph_colour_avx.cpp
2>ojph_transform_avx.cpp
2>Generating Code...
2>C:\FORK\TEMP\OpenJPH\src\core\transform\ojph_transform_avx.cpp(1,1): error C7302: AVX types (__m256) are not currently supported in ARM64EC code
2>C:\FORK\TEMP\OpenJPH\src\core\codestream\ojph_codestream_avx.cpp(1,1): error C7302: AVX types (__m256) are not currently supported in ARM64EC code
2>Done building project "openjph.vcxproj" -- FAILED.
========== Rebuild All: 1 succeeded, 1 failed, 0 skipped ==========
========== Rebuild completed at 11:54 AM and took 09.063 seconds ==========

Type of change

• Excludes _M_ARM64EC from Windows x86‑64 architecture detection in MSVC builds.
• Allows ARM64EC (and ARM64) builds to use the existing macro "OJPH_ARCH_ARM" implementation.

The #else block applies to Linux/FreeBSD/OpenBSD builds and is therefore never selected for Windows ARM64 or ARM64EC.
While _M_ARM64EC is not strictly required here, it is included alongside _M_ARM64 for consistency with the existing ARM64 detection logic.

#else  // Linux/FreeBSD/OpenBSD

  #if defined(__aarch64__) || defined(_M_ARM64) || defined(_M_ARM64EC) // 64-bit ARM

Impact

• Fixes compilation errors for Windows ARM64EC builds.
• No behavior changes for existing x86 or x64 builds.

Build & Test

• Environment: Windows 11 on ARM64; MSVC (VS 2022 v143 toolset).
• Result: ARM64 and ARM64EC configurations compile cleanly; x86/x64 unchanged.

[aous72]

Thank you navvyswethgraphics for this PR.