Error on ld output file name matching input file name

It's not foolproof, for example we don't catch output to a linker
script, to a library specified with -l, or to an element of a thin
archive.

	* ldlang.c (open_output): Exit with error on output file matching
	an input file.
	* testsuite/ld-misc/just-symbols.exp: Adjust ld -r test to suit.