jemalloc: set LG_VADDR to 64 on amd64

and allow to revert it back to 48 with WITHOUT_JEMALLOC_LG_VADDR_WIDE build
option.

Reviewed by: andrew, emaste
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D46686