linux(4): Fixup the vDSO initialization order.

The vDSO initialisation order should be as follows:

• native abi init via exec_sysvec_init();
• vDSO symbols queued to the linux_vdso_syms list;
• linux_vdso_install();
• linux_exec_sysvec_init();

As the exec_sysvec_init() called with SI_ORDER_ANY (last) at SI_SUB_EXEC
order, move linux_vdso_install() and linux_exec_sysvec_init() to the
SI_SUB_EXEC+1 order.

Reviewed by: trasz
Differential Revision: https://reviews.freebsd.org/D30902
MFC after 2 weeks