Use target_endian, not target.arch in cabi_powerpc64

Now target_arch is powerpc64 on both big and little endian, we need to
use target_endian when there are differences in the two ABIs.

Author: antonblanchard

src/librustc_trans/trans/cabi_powerpc64.rs

@@ -158,7 +158,7 @@ fn classify_ret_ty(ccx: &CrateContext, ty: Type) -> ArgType {
     }
 
     // The PowerPC64 big endian ABI doesn't return aggregates in registers
-    if ccx.sess().target.target.arch == "powerpc64" {
+    if ccx.sess().target.target.target_endian == "big" {
         return ArgType::indirect(ty, Some(Attribute::StructRet))
     }