linux(4): Fixup the vDSO initialization order.

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