riscv: Conditionally modify the ELF64 sysentvec for SV48

A sysinit determines whether the pmap has enabled SV48 mode and modifies
the corresponding fields which describe the user memory map.

Reviewed by: kib, jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ecaf115434cf5b427339d27860bcbee3e9f48e39)