Support a wider history counter in pmcstat(8) gmon output

pmcstat(8) contains an option to output sampling data in a gmon format
compatible with gprof(1). Currently, it uses the default histcounter,
which is an (unsigned short). With large sets of sampling data, it
is possible to overflow the maximum value provided by an (unsigned
short).

This change adds the -e argument to pmcstat. If -e and -g are both
specified, pmcstat will use a histcounter type of uint64_t.

Differential Revision: https://reviews.freebsd.org/D4151
Reviewed by: jhb, bjk
Approved by: gnn (mentor)
MFC after: 1 month
Sponsored by: Juniper Networks