Tests: check for target availability for target-specific tests.

Lots of tests are using an explicit target triple w/o first checking that the
target is actually available. Add a REQUIRES clause to a bunch of them. This should
hopefully unbreak bots which don't configure w/ all targets enabled.

llvm-svn: 159949
diff --git a/clang/test/CodeGen/asm_arm.c b/clang/test/CodeGen/asm_arm.c
index 633bf55..4b764b7 100644
--- a/clang/test/CodeGen/asm_arm.c
+++ b/clang/test/CodeGen/asm_arm.c
@@ -1,3 +1,4 @@
+// REQUIRES: arm-registered-target
 // RUN: %clang_cc1 -triple armv6-unknown-unknown -emit-llvm -o - %s | FileCheck %s
 
 void test0(void) {