Explicitly set armv7-eabi target triple on ARM.

This lets the compiler generate FMA instructions, which seems
desirable.

ispc.cpp

@@ -576,6 +576,10 @@ Target::SupportedTargetISAs() {
 std::string
 Target::GetTripleString() const {
     llvm::Triple triple;
+    if (m_arch == "arm") {
+        triple.setTriple("armv7-eabi");
+    }
+    else {
         // Start with the host triple as the default
         triple.setTriple(llvm::sys::getDefaultTargetTriple());
 
@@ -591,7 +595,7 @@ Target::GetTripleString() const {
             triple.setArchName("x86_64");
         else
             triple.setArchName(m_arch);
-
+    }
     return triple.str();
 }