Add basic support for Windows ARM64EC

Make libdeflate build for ARM64EC, and add ARM64EC to GitHub Actions.

ARM64EC pretends to be x86_64, but lots of x86_64 functionality is
broken:

- 256-bit and 512-bit vectors are unsupported
- _mm*_load*() do not accept void pointers
- _m_prefetchw() is not supported
- bmi2 intrinsics are not supported

For now just do the simple thing and treat ARM64EC as a generic
architecture, so do not try to use any of the x86_64 optimized code.

The premise of optimizing for ARM64EC seems questionable, after all,
given that proper ARM64 builds are fully supported.

Author: ebiggers

.github/workflows/ci.yml

@@ -139,8 +139,11 @@ jobs:
     name: Build (Windows, Visual Studio ${{matrix.toolset}}, ${{matrix.platform}})
     strategy:
       matrix:
-        platform: [ARM64]
+        platform: [ARM64, ARM64EC]
         toolset: [v143, ClangCL]
+        exclude: # Exclude unsupported combinations
+        - platform: ARM64EC
+          toolset: ClangCL
     runs-on: windows-latest
     steps:
     - uses: actions/checkout@v4
@@ -149,7 +152,7 @@ jobs:
     - run: >
         cmake -B build -G "Visual Studio 17 2022" -T ${{matrix.toolset}}
         -A ${{matrix.platform}} -DCMAKE_C_FLAGS="/W4 /WX"
-        -DCMAKE_INSTALL_PREFIX=build\install
+        -DCMAKE_INSTALL_PREFIX=build\install -DCMAKE_SYSTEM_VERSION=10
     - run: cmake --build build --verbose --config Release
     - run: cmake --install build --verbose --config Release
 

common_defs.h

@@ -67,7 +67,8 @@
 #undef ARCH_ARM32
 #undef ARCH_RISCV
 #ifdef _MSC_VER
-#  if defined(_M_X64)
+   /* Way too many things are broken in ARM64EC to pretend that it is x86_64. */
+#  if defined(_M_X64) && !defined(_M_ARM64EC)
 #    define ARCH_X86_64
 #  elif defined(_M_IX86)
 #    define ARCH_X86_32