imgact_elf: Disable the stack gap for now

The integration with RLIMIT_STACK is still causing problems for some
programs such as lang/sdcc and syzkaller's executor. Until this is
resolved by some work currently in progress, disable the stack gap by
default.

PR: 260303
Reviewed by: kib, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33438