This adds a test that debug/dwarf can read the skeleton DWARF data
from a split DWARF image (though it doesn't currently support piecing
the external DWARF data back together). This should work because
there's nothing particularly different about skeleton DWARF data, but
previously failed because of poor handling of unrecognized attributes.

Updates #12592.

Change-Id: I2fc5f4679883b05ebd7ec9f0b5c398a758181a32
Reviewed-on: https://go-review.googlesource.com/14542

Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: jcd . <jcd@golang.org>