ld/testsuite: don't output to /dev/null

ld/testsuite: don't output to /dev/null

Mingw doesn't have /dev/null and thus "-o /dev/null" will fail.
Currently, all the options are checked using this "-o /dev/null",
resulting in them being disabled on mingw hosts.
Fix that by outputting to a real file for all targets.

ld/ChangeLog:

	* testsuite/config/default.exp: Replace "-o /dev/null" by a
	file.