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
