Workaround the lack of _CALL_SYSV with clang on powerpc

Index: src/qemu/tcg/ppc/tcg-target.inc.c
--- src/qemu/tcg/ppc/tcg-target.inc.c.orig
+++ src/qemu/tcg/ppc/tcg-target.inc.c
@@ -25,6 +25,11 @@
 #include "elf.h"
 #include "../tcg-pool.inc.c"
 
+/* clang does not define _CALL_* */
+#if defined __clang__ && defined __ELF__
+#define _CALL_SYSV 1
+#endif
+
 #if defined _CALL_DARWIN || defined __APPLE__
 #define TCG_TARGET_CALL_DARWIN
 #endif
