MAINTAINERS: Update execve entry with more details

The UAPI elf.h header was missed in the original MAINTAINER entry. Add
it. Include linux-mm mailing list since that's where execve has
traditionally been discussed. Note that this area is Supported, and aim
at the git tree.

Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Kees Cook <keescook@chromium.org>