Remove hacks for old kernels for architectures which require new kernels

* defs.h: Do not define TCB_WAITEXECVE for AARCH64.
* util.c (change_syscall): For AARCH64 and X32, replace code
with dummy "return 0" and a comment explaining why that is ok
for these architectures.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/defs.h b/defs.h
index 50620b3..068ee3d 100644
--- a/defs.h
+++ b/defs.h
@@ -442,7 +442,7 @@
  || defined(HPPA) \
  || defined(SH) || defined(SH64) \
  || defined(S390) || defined(S390X) \
- || defined(ARM) || defined(AARCH64) \
+ || defined(ARM) \
  || defined(MIPS)
 /* This tracee has entered into execve syscall. Expect post-execve SIGTRAP
  * to happen. (When it is detected, tracee is continued and this bit is cleared.)