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

Previous patch truncating the value to 32-bits before taking the popcount.
Upstream made correct patch:
https://github.com/ARM-software/astc-encoder/issues/468#issuecomment-2119169136
This patch will be included in next release of the astc-encoder 4.9.
Also we can apply this patch to misc/astc-encoder (4.7 in ports).

PR: 278722
Fixes: 656401a (fix build on i386 with CPUTYPE=nehalem and newer)