[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/arm-linux-tdep.c

@@ -1629,7 +1629,9 @@ arm_canonicalize_syscall (int syscall)
     case 363: return gdb_sys_rt_tgsigqueueinfo;
     case 364: return gdb_sys_perf_event_open;
     case 365: return gdb_sys_recvmmsg;
+      */
     case 366: return gdb_sys_accept4;
+      /*
     case 367: return gdb_sys_fanotify_init;
     case 368: return gdb_sys_fanotify_mark;
     case 369: return gdb_sys_prlimit64;