[gdb/record] Support recording syscall accept4

While reviewing the enum gdb_syscall entries with values >= 500, I noticed
that gdb_sys_accept exists, but gdb_sys_accept4 doesn't, while recording
support is essentially the same, given that the difference in interface is
only an extra int parameter:
...
int accept (int sockfd, struct sockaddr *addr, socklen_t *addrlen);
int accept4 (int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags);
...

Fix this by:
- adding gdb_sys_accept4,
- supporting it in record_linux_system_call alongside gdb_sys_accept, and
- mapping to gdb_sys_accept4 in various syscall canonicalization functions.

The usual thing to do before the rewrite of i386_canonicalize_syscall would
have been to use the value from arch/x86/entry/syscalls/syscall_32.tbl:
...
  gdb_sys_accept4 = 364,
...
but that's no longer necessary, so instead we use some >= 500 value:
...
  gdb_sys_accept4 = 533,
...
to steer clear of the space where ppc_canonicalize_syscall and
s390_canonicalize_syscall do hard-coded number magic.

Tested on x86_64-linux, with and without target board unix/-m32, and
aarch64-linux.

Approved-By: Guinevere Larsen <guinevere@redhat.com>

gdb/amd64-linux-tdep.c

@@ -556,6 +556,10 @@ amd64_canonicalize_syscall (enum amd64_syscall syscall_number)
   case amd64_x32_sys_accept:
     return gdb_sys_accept;
 
+  case amd64_sys_accept4:
+  case amd64_x32_sys_accept4:
+    return gdb_sys_accept4;
+
   case amd64_sys_sendto:
   case amd64_x32_sys_sendto:
     return gdb_sys_sendto;