fix PR6584: __SSE3__ not defined with -mss3

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98342 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 3b226d0..8f472b3 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -935,6 +935,7 @@
       .Case("sse42", SSE42)
       .Case("sse41", SSE41)
       .Case("ssse3", SSSE3)
+      .Case("sse3", SSE3)
       .Case("sse2", SSE2)
       .Case("sse", SSE1)
       .Case("mmx", MMX)