ld: Add additional checking for warnings/errors in testsuite

ld: Add additional checking for warnings/errors in testsuite

This commit adds new actions to the run_cc_link_tests mechanism in the
linker testsuite.

Previously this procedure could take a parameter containing a regular
expression that would be matched against warnings from the linker.

After this commit the warnings parameter is removed, instead, the
actions list can contain the actions 'warning', 'error',
'warning_output', or 'error_output'.  The action names are chosen to
match the actions already present in the run_dump_test procedure.

These new actions allow for the current warning checking, but also allow
for checking of errors using a regular expression.  More interestingly,
the *_output actions allow for the warning/error patterns to be placed
in a separate file.

The small number of tests that make use of the warning parameter have
been updated to the new mechanism.  Later commits will make use of the
new features added in this commit.

ld/ChangeLog:

	* testsuite/lib/ld-lib.exp (run_cc_link_tests): Add warning,
	error, warning_output, and error_output actions.  Remove separate
	warnings parameter.
	* testsuite/ld-elf/shared.exp (build_tests): Updated to use
	'warning' action.
	* testsuite/ld-plugin/lto.exp (lto_link_tests): Likewise.