When testing targets for multiple multilibs, it's nice to be able to
run tests in parallel, on a single box or even on multiple boxes that
share the build tree.  This patch introduces check//% targets, similar
to check-gcc//% targets in GCC, that enable tests to be run in
parallel, each one using a separate testsuite directory for the
binaries and logs.

Such pattern rules require GNU make to work, and are silently ignored
by other makes (unless you happen to run check//%, which will still
work, but run the testsuite with --target_board=% :-)

Ok to install?