The fileNM structure does not need it as NM symbolization does not use the field.

* Remove unnecessary memoization.

* Handle large mappings in FindProgHeaderForMapping.

In some cases, the loader maps the entire program address space in a single
runtime mapping that starts from the file offset of the first segment but
encompasses additional segments. Such mappings may be split later when
samples are detected in the following segments. However, a perf.data file
still includes the large initial mapping, which should be associated with
the first segment both before and after an eventual split. For each
segment, we compute the aligned file offset of its mapping assuming 4k page
alignment, and we filter out program segments that have aligned file
offsets greater than the given mapping file offset.

Co-authored-by: Alexey Alexandrov <aalexand@users.noreply.github.com>

…14. (#615)

* Add testing for Go Tip and Go 1.16

* Add 1.16 support for Linux

By default, a numeric label with a value of zero is interpreted as an
empty label which will be ignored during parsing. However, a zero value
can be a valid label value. Instead of ignoring it altogether, parse it
as a zero-valued numeric label if num_unit is set.

This change restores the old behavior in pprof of always  selecting
the executable segment  when computing the base for a mapping.

This is a temporary rollback, while we fix issues with the new segment
matching heuristic.