amd64 pmap: Prefer consistent naming for loader tunable

The sysctl knob 'vm.pmap.allow_2m_x_ept' is loader tunable and have
public document entry in security(7) but is fetched from kernel
environment 'hw.allow_2m_x_ept'. That is inconsistent and obscure.

As there is public security advisory FreeBSD-SA-19:25.mcepsc [1],
people may refer to it and use 'hw.allow_2m_x_ept', let's keep old
name for compatibility.

[1] https://www.freebsd.org/security/advisories/FreeBSD-SA-19:25.mcepsc.asc

Reviewed by: kib
Fixes: c08973d09c95 Workaround for Intel SKL002/SKL012S errata
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42311

(cherry picked from commit 9e7f349ff10691c2e3fb03898dbc942794a47566)