testsuite: /dev/null is not accessible on Windows

testsuite: /dev/null is not accessible on Windows

When running the DejaGNU testsuite on a toolchain built for native
Windows, the path /dev/null can't be used to open a stream to void.
On native Windows, the resource is instead named "nul".

The error would look like this:
c:/arm-11.3.rel1/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: cannot find @/dev/null: No such file or directory

Patch has been verified on Windows and Linux.

gcc/testsuite:

	* gcc.misc-tests/outputs.exp: Use "@nul" for Windows,
	"@/dev/null" for other environments.

Co-Authored-By: Yvan ROUX  <yvan.roux@foss.st.com>
Signed-off-by: Torbjörn SVENSSON  <torbjorn.svensson@foss.st.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>