Extract a set of pmcstat functions and interfaces to the new internal
library -- libpmcstat.

This includes PMC logging module, symbols lookup functions,
ELF parsing, process management, PMC attachment, etc.

This allows to reuse code while building new hwpmc(4)-based applications.

Also add pmcstat_symbol_search_by_name() function that allows to find
mapped IP range for a given function name.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12718