fix x86_64 arch detection when cross-compiling on macos arm64

[LudovicoYIN]

Summary

Fix #6505 — when cross-compiling ncnn for x86_64 on macOS arm64 with
-DCMAKE_OSX_ARCHITECTURES=x86_64, the target architecture is incorrectly
detected as ARM, causing all x86 SIMD optimizations (SSE/AVX/AVX2/AVX512)
to be skipped.

Root cause

In CMakeLists.txt:164, CMAKE_SYSTEM_PROCESSOR always reports the host
CPU (arm64), even when CMAKE_OSX_ARCHITECTURES is explicitly set to
x86_64. This causes the ARM arch branch to match, setting
NCNN_TARGET_ARCH to arm instead of x86.

Fix

On Apple platforms, skip the CMAKE_SYSTEM_PROCESSOR arm match when
CMAKE_OSX_ARCHITECTURES is explicitly set to a non-arm architecture.

Before / After

	Before	After
Target arch	arm 64bit	x86 64bit
NEON/VFPv4/SVE	attempted, all FAIL	skipped
SSE/AVX/AVX2/FMA/F16C	skipped	all detected
AVX512/VNNI/BF16/FP16	skipped	all detected
Build output	x86_64 (no SIMD)	x86_64 (full SIMD)

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✅ LudovicoYIN
❌ nihui
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[LudovicoYIN]

@nihui This PR fixes the arch detection issue in #6505. PTAL when you have a moment, thanks!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.97%. Comparing base (866b73d) to head (69065c5).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6730      +/-   ##
==========================================
- Coverage   95.98%   95.97%   -0.01%     
==========================================
  Files         965      965              
  Lines      404957   404634     -323     
==========================================
- Hits       388681   388352     -329     
- Misses      16276    16282       +6     

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[Copilot]

Pull request overview

Fixes incorrect target architecture detection when cross-compiling ncnn for x86_64 on Apple Silicon (macOS arm64) with -DCMAKE_OSX_ARCHITECTURES=x86_64, which previously caused the build to be treated as ARM and skipped all x86 SIMD feature detection.

Changes:

• Adjust ARM-architecture detection on Apple platforms to avoid using CMAKE_SYSTEM_PROCESSOR (host CPU) when CMAKE_OSX_ARCHITECTURES is explicitly set to a non-ARM architecture.
• Ensures x86_64 cross-compiles on macOS arm64 properly follow the x86 feature-probing path (SSE/AVX/etc.).

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[nihui]

LGTM

[LudovicoYIN]

Thanks review.

[nihui]

Thanks for your contribution !