linux-user: fail execve() if env/args too big

linux-user: fail execve() if env/args too big

If the host's page size is equal to or smaller than the target's, native
execve() will fail appropriately with E2BIG if called with too big an
environment for the target to handle. It may falsely succeed, however, if
the host's page size is bigger, and feed the executed target process an
environment that is too big for it to handle, at which point QEMU barfs and
exits, confusing procmail's autoconf script and causing the build to fail.

This patch makes sure that execve() will return E2BIG if the environment is
too large for the target.

Signed-off-by: Ulrich Hecht <uli@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>