test: add an explicit target triple

Now that we support WoA, this test fails on ARM build bots as __va_start has a
different signature on different architectures.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213714 91177308-0d34-0410-b5e6-96231b3b80d8

test/Sema/MicrosoftExtensions.c

-// RUN: %clang_cc1 %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify -fms-extensions
+// RUN: %clang_cc1 -triple i686-windows %s -fsyntax-only -Wno-unused-value -Wmicrosoft -verify -fms-extensions
 
 
 struct A