Mesa: Disable SWR on x86_64 platforms w/o AVX

[geimer]

Trying to build Mesa 21.1.1 on a somewhat aged x86_64 system w/o AVX fails during the configuration step, since the custom Easyblock passes -Dgallium-drivers=swrast,swr -Dswr-arches= to meson (note the empty swr-arches setting). While the SWR driver can be manually disabled by setting the gallium-drivers configuration option, the Easyblock could also automatically fall back to disabling SWR if AVX support is missing on the build platform.