configure/swr: configurable swr architectures

configure/swr: configurable swr architectures

Allow configuration of the SWR architecture depend libraries
we build for with --with-swr-archs.  Maintains current behavior
by defaulting to avx,avx2.

Scons changes made to make it still build and work, but
without the changes for configuring which architectures.

v2:
 * add missing comma for swr_archs default
 * check that at least one architecture is enabled
 * modify loader logic to make it clearer how to add archs

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>