__pcpu: gcc -Wredundant-decls

Pollution from counter.h made pcpu visible in amd64/pmap.c. Delete
the existing extern decl of pcpu in amd64/pmap.c and avoid referring
to that symbol, instead accessing the pcpu region via PCPU_SET macros.
Also delete an unused extern decl of __pcpu from mp_x86.c.

Reviewed by: kib
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11666