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