winebuild: Only allow the stdcall calling convention for system calls.

tools/winebuild/import.c

@@ -1437,7 +1437,7 @@ void output_syscalls( DLLSPEC *spec )
                 output( "\tmovl $%s,%%edx\n", asm_name("__wine_syscall") );
             }
             output( "\tcall *%%edx\n" );
-            output( "\tret $%u\n", odp->type == TYPE_STDCALL ? get_args_size( odp ) : 0 );
+            output( "\tret $%u\n", get_args_size( odp ));
             break;
         case CPU_x86_64:
             /* Chromium depends on syscall thunks having the same form as on

tools/winebuild/parser.c

@@ -315,9 +315,9 @@ static int parse_spec_arguments( ORDDEF *odp, DLLSPEC *spec, int optional )
     }
     if (odp->flags & FLAG_SYSCALL)
     {
-        if (odp->type != TYPE_STDCALL && odp->type != TYPE_CDECL)
+        if (odp->type != TYPE_STDCALL)
         {
-            error( "A syscall function must use either the stdcall or the cdecl convention\n" );
+            error( "A syscall function must use the stdcall convention\n" );
             return 0;
         }
     }