Put vector types in regs for arm & mips FFI.

This seems to match what clang does on arm, but I cannot do any
experimentation with mips, but it matches how the LLVM intrinsics are
defined in any case...

src/librustc_trans/trans/cabi_aarch64.rs

@@ -139,7 +139,8 @@ fn is_reg_ty(ty: Type) -> bool {
         Integer
         | Pointer
         | Float
-        | Double => true,
+        | Double
+        | Vector => true,
         _ => false
     }
 }

src/librustc_trans/trans/cabi_arm.rs

@@ -169,7 +169,8 @@ fn is_reg_ty(ty: Type) -> bool {
         Integer
         | Pointer
         | Float
-        | Double => true,
+        | Double
+        | Vector => true,
         _ => false
     }
 }

src/librustc_trans/trans/cabi_mips.rs

@@ -123,7 +123,8 @@ fn is_reg_ty(ty: Type) -> bool {
         Integer
         | Pointer
         | Float
-        | Double => true,
+        | Double
+        | Vector => true,
         _ => false
     };
 }