Correctly handle the degenerated triple "thumb".

Fixes a crash in llc where some parts think the target is thumb and others think
it is ARM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197607 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/Thumb/triple.ll

@@ -0,0 +1,7 @@
+; RUN: llc < %s -mtriple=thumb | FileCheck %s
+
+; CHECK: .code	16
+
+define void @f() {
+  ret void
+}