Port sysctl kern.elf32.read_exec from amd64 to i386.

Make it more comprehensive on i386, by not setting nx bit for any
mapping, not just adding PF_X to all kernel-loaded ELF segments. This
is needed for the compatibility with older i386 programs that assume
that read access implies exec, e.g. old X servers with hand-rolled
module loader.

Reported and tested by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week