Multi-target testing

[Ben Elliston]

The cpu component test framework needs a bit of work to get it up to
scratch.  I'd like to suggest that if the tree is configured for a
specific target, that $(top_builddir)/gas/as-new is tried for building
test cases.  If all targets are configured, or an assembler cannot be
found in the build tree, we can either skip the tests ("untested") or
run ${exec-prefix}-as from the path.

Is this a reasonable approach?  Any better ideas?

Ben