current FreeBSD signal trampoline, but does not specifies
sv_sigcode_base, since shared page is not mapped. This results in the
zero %eip for the signal frame. Fall back to calculating %eip as
offset from the psstrings when sv_sigcode_base is not initialized.
Reported by: Rich Naill <rich@enterprisesystems.net>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week