`-C split-debuginfo={off,packed,unpacked}` is (effectively) untested on windows-msvc and windows-gnu (well, windows generally)

[jieyouxu]

tests/run-make/split-debuginfo has this:

rust/tests/run-make/split-debuginfo/Makefile

Lines 34 to 38 in 2776bdf

	ifdef IS_WINDOWS
	# Windows only supports packed debuginfo - nothing to test.
	off:
	packed:
	unpacked:

Which lumps windows-msvc and windows-gnu together, and also doesn't test anything on windows altogether.

Noticed while working on porting split-debuginfo to rmake.rs.

If this is tested somewhere else, well, it's clearly not in this run-make test.