LD/testsuite: run_dump_test: Report UNSUPPORTED for excluded targets

LD/testsuite: run_dump_test: Report UNSUPPORTED for excluded targets

Bring the LD implementation of `run_dump_test' in line with its binutils
and GAS counterparts and report the UNSUPPORTED status for tests the run
of which has been prevented by means of one or more of `target',
`alltargets' and `notarget' tags.  Define `skip', `anyskip' and `noskip'
tags, which do not report anything for tests that are not run.

The rationale behind this is that we want to have unsupported tests
reported to ensure that they have actually been attempted and have not
been accidentally suppressed.  Then tests which have target-specific
variants that cannot be expressed with a single .d file can make use of
the newly added tags to silently suppress the uninteresting variants.

	ld/
	* testsuite/lib/ld-lib.exp (run_dump_test): Call `unsupported'
	if the target being tested has been excluded by means of one or
	more of `target', `alltargets' and `notarget' tags.  Add support
	for `skip', `anyskip' and `noskip' tags.