meson: implement default driver arguments

meson: implement default driver arguments

This allows drivers to be set by OS/arch in a sane manner.

v2: - set _drivers to a list of drivers instead of manually assigning
      each with_*
v3: - Use "auto" instead of "default", which matches the value of other
      automatically configured options.
    - Set vulkan drivers as well
    - Add error message if no automatic drivers are known for a given
      arch/OS combo
    - use not(darwin or windows) instead of (linux or *bsd), which is
      probably more accurate (that way Solaris and other *nix systems
      aren't excluded)
    - rename softpipe to swrast, as swrast is the actual option name

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>