ExitStatus unknown wait status test: Make it Linux only

If different unices have different bit patterns for WIFSTOPPED and
WIFCONTINUED then simply being glibc is probably not good enough for
this rather ad-hoc test to work.  Do it on Linux only.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -22,6 +22,6 @@ fn exitstatus_display_tests() {
     // Testing "unrecognised wait status" is hard because the wait.h macros typically
     // assume that the value came from wait and isn't mad.  With the glibc I have here
     // this works:
-    #[cfg(target_env = "gnu")]
+    #[cfg(all(target_os = "linux", target_env = "gnu"))]
     t(0x000ff, "unrecognised wait status: 255 0xff");
 }