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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159949 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/altivec.c b/test/CodeGen/altivec.c
index 2982303..6c924a7 100644
--- a/test/CodeGen/altivec.c
+++ b/test/CodeGen/altivec.c
@@ -1,3 +1,4 @@
+// REQUIRES: ppc32-registered-target
 // RUN: %clang_cc1 -faltivec -triple powerpc-unknown-unknown -emit-llvm %s -o - | FileCheck %s
 
 // Check initialization