dav1d misdetects x32 as amd64

dav1d misdetects x32 as amd64 and uses assembly that was not ported to x32 leading to failures such as:

/usr/bin/ld: i386:x86-64 architecture of input file 'src/25a6634@@dav1d@sha/loopfilter.obj' is incompatible with i386:x64-32 output

Even if this arch is not widely used, the build system should probably detect this arch and disable assembly code.