Fix assert check for SV_DSO_SIG in exec_sysvec_init_secondary()

The only requirement for SV_DSO_SIG here is that the flags match between
the source and target sysentvec.

The current assertion is too strict and fails on powerpc64, the only
other architecture than amd64 that uses this function, which doesn't
implement sigtramp in a VDSO.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D33355