[Tests] Make OS features consistent

lit.py currently allows any substring of `target_triple` to be used as a
feature in REQUIRES/UNSUPPORTED/XFAIL. This results in various forms of
the OS spread across the tests and is also somewhat confusing since they
aren't actually listed in the available features.

Modify all OS-related features to use the `OS=` version that Swift adds
instead. We can later remove `config.target_triple` so that these don't
the non-OS versions don't work in the first place.

test/Driver/parseable_output.swift

@@ -1,6 +1,6 @@
 // RUN: %swiftc_driver_plain -emit-executable %s -o %t.out -emit-module -emit-module-path %t.swiftmodule -emit-objc-header-path %t.h -serialize-diagnostics -emit-dependencies -parseable-output -driver-skip-execution 2>&1 | %FileCheck %s
 
-// XFAIL: freebsd, openbsd, linux
+// XFAIL: OS=freebsd, OS=openbsd, OS=linux-gnu
 
 // CHECK: {{[1-9][0-9]*}}
 // CHECK-NEXT: {