Skip x64 asm objects for non-x86_64 builds

[carlosqwqqwq]

Why

rtpengine already has conservative non-x86_64 fallbacks in its codec and mixing code, but the build system still unconditionally adds several x64 assembly objects to lib and daemon.

For riscv64, that means the source-level generic path exists, but the makefile plan still tries to carry mix_in_x64_* and mvr2s_x64_* objects into the build.

This patch keeps the existing x86_64 fast path intact and only makes the object selection follow the compiler target architecture.

What changed

• Updated lib/Makefile:
  • detect the compiler target architecture with $(CC) -dumpmachine;
  • only add mix_in_x64_* and mvr2s_x64_* assembly objects when the target architecture is x86_64.
• Updated daemon/Makefile:
  • use the same compiler-target detection;
  • only add transcoding-related x64 assembly objects when the target architecture is x86_64.

Verification

• Verified the intended source-level behavior before touching the build scripts:
  • lib/codeclib.c and lib/mix_buffer.c already keep x86-specific SIMD dispatch behind __x86_64__ guards and provide generic fallbacks elsewhere.
• Verified the patched build plans in Docker (ubuntu:24.04) with make -pn:
  • with host gcc, lib/Makefile still reports CC_ARCH := x86_64 and keeps ASM := mix_in_x64_* mvr2s_x64_*;
  • with host gcc, daemon/Makefile still reports CC_ARCH := x86_64 and keeps LIBASM := mvr2s_x64_* mix_in_x64_*;
  • with CC=riscv64-linux-gnu-gcc, lib/Makefile reports CC_ARCH := riscv64 and no longer includes any mix_in_x64_* or mvr2s_x64_* assembly objects;
  • with CC=riscv64-linux-gnu-gcc, daemon/Makefile reports CC_ARCH := riscv64 and no longer includes any x64 assembly objects;
  • a final scan over both make -pn outputs confirmed no mix_in_x64 or mvr2s_x64 objects remain in the riscv64 build plan.
• Verified the object-selection behavior by actually executing the patched daemon/Makefile rules in Docker (ubuntu:24.04) with a fake compiler harness:
  • when the fake compiler reports riscv64-unknown-linux-gnu, the rtpengine target builds without touching any mix_in_x64_* or mvr2s_x64_* assembly objects;
  • when the same harness reports x86_64-linux-gnu, the rtpengine target still compiles mvr2s_x64_avx2.S, mvr2s_x64_avx512.S, mix_in_x64_avx2.S, mix_in_x64_avx512bw.S, and mix_in_x64_sse2.S;
  • this confirms the patch changes the actual object-selection boundary, not just the printed plan.

Notes

• This is a conservative portability patch. It does not add RVV or any RISC-V-specific optimized backend.
• The goal is to stop non-x86_64 targets from inheriting x64 assembly objects when the generic C path is already the correct fallback.