Add PLATFORM key to lit.cfg

This is less cumbersome to use than combinations of OS= or somesuch
when XFAIL-ing tests that only fail on a particular platform or
using objc_interop to mean Apple platforms.

Author: bitjammer

test/lit.cfg

@@ -430,6 +430,7 @@ for target in config.targets_to_build.split():
 # Add the run target CPU, OS, and pointer size as features.
 config.available_features.add("CPU=" + run_cpu)
 config.available_features.add("OS=" + run_os)
+config.available_features.add("PLATFORM=" + platform.system())
 config.available_features.add("PTRSIZE=" + run_ptrsize)
 
 if run_cpu == "i386" or run_cpu == "x86_64":