x86: whack pmspcv from GENERIC

The driver is enormous and rarely used.

    text      data       bss        dec         hex   filename
23076646   1870505   4415872   29363023   0x1c00b4f   kernel.before
20017433   1870305   4416000   26303738   0x1915cfa   kernel.after

People using the driver will need to add pmspcv_load="YES" to
their loader.conf.

Reviewed by: jhb
Relnotes: yes
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39816