gallium/swr: fold the almost identical Makefiles

Rather than having two almost identical Makefiles, with various VPATH
hacks just fold them, using COMMON_* variables and actually getting
things buildable/shipable.

v2: whitespace fixes, remove Makefile.sources-arch

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>
7 files changed