Invalidate cache for the PDPTE page when using PAE paging but PAT is

Invalidate cache for the PDPTE page when using PAE paging but PAT is
not supported.

According to SDM rev. 69 vol. 3, for PDPTE registers loads:
- when PAT is not supported, access to the PDPTE page is performed as
  UC, see 4.9.1;
- when PAT is supported, the access is WB, see 4.9.2.

So potentially CPU might load stale memory as PDPTEs if both PAT and
self-snoop are not implemented.  To be safe, add total local cache
flush to pmap_cold() before initial load of cr3, and flush PDPTE page
in pmap_pinit(), if PAT is not implemented.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D19365