treewide: Allow additional system calls for i386/i686

I haven't tested i386 for a long time (after playing with some
openSUSE i586 image a couple of years ago). It turns out that a number
of system calls we actually need were denied by the seccomp filter,
and not even basic functionality works.

Add some system calls that glibc started using with the 64-bit time
("t64") transition, see also:

  https://wiki.debian.org/ReleaseGoals/64bit-time

that is: clock_gettime64, timerfd_gettime64, fcntl64, and
recvmmsg_time64.

Add further system calls that are needed regardless of time_t width,
that is, mmap2 (valgrind profile only), _llseek and sigreturn (common
outside x86_64), and socketcall (same as s390x).

I validated this against an almost full run of the test suite, with
just a few selected tests skipped. Fixes needed to run most tests on
i386/i686, and other assorted fixes for tests, are included in
upcoming patches.

Reported-by: Uroš Knupleš <uros@knuples.net>
Analysed-by: Faidon Liambotis <paravoid@debian.org>
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1078981
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tcp.c

@@ -2144,7 +2144,7 @@ cancel:
  * @c:		Execution context
  * @ref:	epoll reference of timer (not connection)
  *
- * #syscalls timerfd_gettime
+ * #syscalls timerfd_gettime i686:timerfd_gettime64
  */
 void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
 {