Add support for llvm-gsymutil

[sfc-gh-sgiesecke]

Symbolication is supported through addr2line, llvm-symbolizer and nm right now. When GSYM data is available for a binary, using llvm-gsymutil would be much faster and less memory-intensive than addr2line or llvm-symbolizer.

To make this fit in well with the execution model of pprof, it requires an extension of llvm-gsymutil to allow it to be fed the input addresses from stdin, as suggested by https://reviews.llvm.org/D102224

[nolanmar511]

Per discussion in #636, this is something we don't think makes sense to support directly within pprof at this time.

Marking as closed.