Skip linker-output-non-utf8 test on Apple
This test fails on APFS filesystems with the following error:
```shell
mkdir: /Users/ryan/Code/rust/build/x86_64-apple-darwin/test/run-make/linker-output-non-utf8.stage2-x86_64-apple-darwin/zzz�: Illegal byte sequence
```
The mkdir does succeed on an HFS+ volume mounted on the same system:
```shell
$ mkdir zzz$$'\xff'
$ ls
zzz47432\xff
```
This is due to APFS now requiring that all paths are valid UTF-8. As APFS will be the default filesystem for all new Darwin-based systems the most straightforward fix is to skip this test on Darwin as well as Windows.