Modify exec_sysvec_init() to allow non-native abi to setup their sysentvecs.

For future use in the Linux emulation layer modify the exec_sysvec_init()
to allow non-native abi to fill sv_timekeep_base and sv_shared_page_obj.

Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D30898
MFC after: 2 weeks