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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34274