syscalls,x86: implement execveat() system call

Add a new system execveat(2) syscall. execveat() is to execve() as
openat() is to open(): it takes a file descriptor that refers to a
directory, and resolves the filename relative to that.

In addition, if the filename is empty and AT_EMPTY_PATH is specified,
execveat() executes the file to which the file descriptor refers. This
replicates the functionality of fexecve(), which is a system call in
other UNIXen, but in Linux glibc it depends on opening
"/proc/self/fd/<fd>" (and so relies on /proc being mounted).

The filename fed to the executed program as argv[0] (or the name of the
script fed to a script interpreter) will be of the form "/dev/fd/<fd>"
(for an empty filename) or "/dev/fd/<fd>/<filename>", effectively
reflecting how the executable was found.  This does however mean that
execution of a script in a /proc-less environment won't work.

Only x86-64, i386 and x32 ABIs are supported in this patch.

Based on patches by Meredydd Luff <meredydd@senatehouse.org>

Signed-off-by: David Drysdale <drysdale@google.com>
syscalls,x86: add selftest for execveat(2)

Signed-off-by: David Drysdale <drysdale@google.com>