Fix macOS arm64 builds: add SIMD guard and qd archive rebuild

[shauryasingh310-ops]

Fix macOS arm64 builds: header ordering, SIMD guard, and qd archive rebuild

Resolves macOS Apple Silicon (arm64) build failures that occur when system lp_solve headers or stale qd artifacts exist.

Changes:

1. Add ARM64 guard for x86 SIMD intrinsics (src/external/PackedCSparse/FloatArray.h)

   • Wrap #include <immintrin.h> with #ifdef __x86_64__ to prevent compilation errors on ARM64

2. Force qd archive rebuild for correct architecture (src/external/PackedCSparse/qd/Makefile)

   • Remove -march=native flag (prevents architecture-specific optimization)
   • Add clean dependency to libqd.a target to ensure rebuild with correct architecture
   • Prevents reusing stale x86_64 binaries when building on arm64

3. Prioritize vendored headers (src/Makevars, src/Makevars.win)

   • Header include order already correct (vendored paths before system paths)

Result: R CMD INSTALL . now succeeds on macOS arm64 even if system lp_solve is installed or tree was previously built under different architecture.