Bug 1576710 - Perform the correct comparison with OS Arch r=nalexander

This removes an unrecognized parameter warning.

Differential Revision: https://phabricator.services.mozilla.com/D43504

--HG--
extra : moz-landing-system : lando

build/clang-plugin/moz.build

@@ -73,7 +73,7 @@ if CONFIG['HOST_OS_ARCH'] == 'Darwin':
 
 # As of clang 8, llvm-config doesn't output the flags used to build clang
 # itself, so we don't end up with -fPIC as a side effect. llvm.org/PR8220
-if CONFIG['HOST_OS_ARCH'] != 'Windows':
+if CONFIG['HOST_OS_ARCH'] != 'WINNT':
     HOST_CXXFLAGS += ['-fPIC']
 
 DIRS += [