Add nasm/yasm on Intel only?

[Craig Treleaven]

Hi:

With Apple Silicon macs taking over the world (eventually), I noticed that a 
few ports build on the new platform but still declare a dependency on the nasm 
or yasm assemblers.  Obviously these aren’t used when building for arm64.  What 
would be a clean, simple way of only adding the dependency when building for 
Intel archs?  Something along the lines of:

if {{build_arch} in "i386 x86_64" || [variant_isset universal] } {
    depends_build-append  port:nasm
}

Craig