swr: (autoconf) allow a single swr architecture to be builtin

swr: (autoconf) allow a single swr architecture to be builtin

Part 1 of 2 (part 1 is autoconf changes, part 2 is C++ changes)

When only a single SWR architecture is being used, this allows that
architecture to be builtin rather than as a separate libswrARCH.so that
gets loaded via dlopen.  Since there are now several different code
paths for each detected CPU architecture, the log output is also
adjusted to convey where the backend is getting loaded from.

This allows SWR to be used for static mesa builds which are still
important for large HPC environments where shared libraries can impose
unacceptable application startup times as hundreds of thousands of copies
of the libs are loaded from a shared parallel filesystem.

Based on an initial implementation by Tim Rowley.

v2: Fix comment placement pointed out by Bruce C.

Signed-off-by: Chuck Atkins <chuck.atkins@kitware.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>
CC: Tim Rowley <timothy.o.rowley@intel.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>