graphics/bgfx: fix build on i386 with CPUTYPE=nehalem and newer

Build error:
/wrkdirs/usr/ports/graphics/bgfx/work/bgfx.cmake-1.127.8725-465/bimg/3rdparty/astc-encoder/source/astcenc_vecmathlib_sse_4.h:1313:26: error: use of undeclared identifier '_mm_popcnt_u64'; did you mean '_mm_popcnt_u32'?

return static_cast<int>(_mm_popcnt_u64(v));
                        ^~~~~~~~~~~~~~
                        _mm_popcnt_u32

/usr/lib/clang/14.0.5/include/popcntintrin.h:33:1: note: '_mm_popcnt_u32' declared here
_mm_popcnt_u32(unsigned int __A)
^
1 error generated.

Upstream issues:
https://github.com/ARM-software/astc-encoder/issues/468
https://github.com/bkaradzic/bgfx/pull/2929#issuecomment-2093377116

PR: 278722
Approved by: yuri (maintainer timeout, 15 days)