Embree does not build for win-arm64 (upstream toolchain gaps)

[jcant0n]

win-arm64 was dropped from .github/workflows/embree-cmake.yml and from the supported
platform list. Two different walls, both upstream, on Embree v4.4.1:

MSVC toolset. common/sys/intrinsics.h gates its ARM path on __ARM_NEON and
__aarch64__, which are GCC/Clang predefines. MSVC defines neither on ARM64 (it defines
_M_ARM64), so the #else branch runs and pulls in immintrin.h, which errors with
"This header is specific to X86, X64, ARM64, and ARM64EC targets". Note this happens at
compile time: CMake configures perfectly and reports Building for AArch64, so a check on
the configure output is not enough to catch it.

clang-cl toolset (-T ClangCL, clang 19.1.5 ships on the windows-11-arm runner).
Clears the above, then fails with
clang-cl : error : unsupported option '-msse2' for target 'arm64-pc-windows-msvc' on
common/sys, common/math, common/simd and common/tasking — Embree's own CMake handing
x86 flags to an ARM target.

common/cmake/clang.cmake has an ARM branch that omits -msse2, so the flags are coming from
the non-ARM path; working out exactly why, and fixing it, means patching Embree's build system
and carrying that patch here.

Worth weighing before picking this up again: Embree is a ray tracing kernel, and a build forced
through a configuration upstream does not test fails by returning wrong intersections, not by
crashing. Any win-arm64 binary produced this way needs validating against results from a
supported platform before it ships.

Reproduce with gh workflow run embree-cmake.yml -f only-rid=win-arm64 after re-adding the
matrix entry.

Runs: 31488732222 (MSVC), 31493683821 (clang-cl).