fix: guard platform-specific sources for cross-arch/cross-os builds

[manuschillerdev]

This is my first contribution and I am happy to accept any guidance or feedback to make this mergable.

Building with zig build -Dtarget=aarch64-macos fails with three distinct errors. Tested on Zig 0.16, macOS 15 (arm64).

• x86_64 asm compiled on ARM: crypto/bn/asm/x86_64-gcc.c is in the unconditional source list, causing compilation failures on non-x86_64 targets
• Missing arm_arch.h include: ARM crypto sources include arm_arch.h but upstream.path("crypto") wasn't in the include paths, only b.path("crypto") (the local overlay)
• loongarchcap.c on non-Linux: includes sys/auxv.h which only exists on Linux, breaks macOS and other non-Linux builds

Test with:

zig build (native aarch64-macos) # passes
zig build -Dtarget=x86_64-linux-gnu # passes