ld: Add file based error/warning regexp for run_dump_test.

ld: Add file based error/warning regexp for run_dump_test.

The run_dump_test mechanism supports options error and warning, which
allow regexp to be specified within the test file, these regexp are
matched against the stderr output from the linker.

Similar dump test style functions for gas and gdb also support file
based matching versions of these options; the patterns are placed into a
file which the test driver then references.  It is sometimes clearer,
when the pattern to be matched spans multiple lines if the patterns can
be placed into a separate file.

This patch adds new options error_output and warning_output for the
linker run_dump_test function.  These new options take a filename
parameter, this file is then used (with regexp_diff) to compare against
the linker output.

ld/testsuite/ChangeLog:

	* ld/ld-lib.exp (run_dump_test): Add error_output and
	warning_output options.