Fix compiler error: pass -mpopcnt when using clang / llvm and targeting x86_64

[TCROC]

Introduced by PR: #59595

The error occurs when compiling with llvm on linux. At the very least, with the llvm compiler bundled with zig that we use.

In file included from thirdparty/embree/kernels/bvh/bvh_intersector_hybrid4_bvh4.cpp:4:
In file included from thirdparty/embree/kernels/bvh/bvh_intersector_hybrid.cpp:4:
In file included from thirdparty/embree/kernels/bvh/bvh_intersector_hybrid.h:6:
In file included from thirdparty/embree/kernels/bvh/bvh.h:7:
In file included from thirdparty/embree/kernels/bvh/bvh_node_aabb.h:6:
In file included from thirdparty/embree/kernels/bvh/bvh_node_base.h:6:
In file included from thirdparty/embree/kernels/bvh/bvh_node_ref.h:6:
In file included from thirdparty/embree/kernels/bvh/../common/default.h:8:
In file included from thirdparty/embree/kernels/bvh/../common/../../common/sys/thread.h:7:
In file included from thirdparty/embree/kernels/bvh/../common/../../common/sys/mutex.h:7:
thirdparty/embree/kernels/bvh/../common/../../common/sys/intrinsics.h:489:12: error: always_inline function '_mm_popcnt_u64' requires target feature 'popcnt', but would be inlined into function 'popcnt' that is compiled without support for 'popcnt'
  489 |     return _mm_popcnt_u64(in);
      |            ^
1 error generated.l.cpp
scons: *** [bin/obj/thirdparty/embree/kernels/bvh/steambvh_intersector_hybrid4_bvh4.linuxbsd.editor.double.x86_64.llvm.steam.o] Error 1
scons: building terminated because of errors.
INFO: Time elapsed: 00:02:22.99

The fix is to pass the -mpopcnt flag to the compiler so that the function _mm_popcnt_u64 supports always_inline.

[Calinou]

Tested locally, it works as expected (although I can build Godot fine with Clang without that PR on Fedora 42).

This makes sense to do as POPCNT is not mandated by SSE 4.2, but all CPUs that support SSE 4.2 also support POPCNT.

[Repiteo]

Thanks!