Select the proper x86_64 ABI based first and foremost on the specified calling convention instead of just looking at the selected platform

Author: CensoredUsername

src/librustc_trans/abi.rs

@@ -484,7 +484,9 @@ impl FnType {
 
         match &ccx.sess().target.target.arch[..] {
             "x86" => cabi_x86::compute_abi_info(ccx, self),
-            "x86_64" => if ccx.sess().target.target.options.is_like_windows {
+            "x86_64" => if abi == Abi::SysV64 {
+                cabi_x86_64::compute_abi_info(ccx, self);
+            } else if abi == Abi::Win64 || ccx.sess().target.target.options.is_like_windows {
                 cabi_x86_win64::compute_abi_info(ccx, self);
             } else {
                 cabi_x86_64::compute_abi_info(ccx, self);