ld/testsuite: adjust to Windows path separator.

ld/testsuite: adjust to Windows path separator.

In some tests, the path reported on Windows will have a \ instead of a
/. This occurs when a file is concatened with the search path in
ldfile.c.:  "ld -Ltmpdir -ltext" will result into "tmpdir\libtext.a".

ld/ChangeLog:

	* testsuite/ld-elf/retain5.map: Allow \ path separator.
	* testsuite/ld-plugin/plugin-10.d: Likewise.
	* testsuite/ld-plugin/plugin-11.d: Likewise.
	* testsuite/ld-plugin/plugin-18.d: Likewise.
	* testsuite/ld-plugin/plugin-19.d: Likewise.
	* testsuite/ld-plugin/plugin-20.d: Likewise.
	* testsuite/ld-plugin/plugin-22.d: Likewise.