amd64: re-enable la57, try two

If you need to use libc with jemalloc without commit
eade2001aa9d91440886de8359a4dec9edcde2a9 (head, or
b90d68c9b28b0a458bdf4a963397817974ed0f90 stable/14), you have to set
vm.pmap.la57 to 0 for system-global effect, or use proccontrol -m la48
on affected image.

Sponsored by: The FreeBSD Foundation