Make symbols script work cross-platform

[christianbundy]

The symbols script had darwin-x64 hardcoded as the platform and arch,
which means that running the script doesn't work on Linux.

This commit adds directory read and selects a file that starts with
'libsodium', which is a .dylib file on MacOS and .so on Linux. This
commit also makes a slight changes to the nm execution, which was
previously depending on flags that aren't used across other versions of
nm.

This commit uses only --dynamic, and uses the symbol type column to
select the symbols that we want without depending on command-line flags
that aren't always available. I've output the results into the readme,
and the only surprising change is that numbers and underscores are now
being sorted differently in the 'missing altogether' section. This is
because the previous code sorted the unparsed lines from nm, which meant
that we were sorting by memory address rather than symbol name.

[emilbayes]

I think it should be looking for .so on linux, as it's trying to read symbols from libsodium not the bindings :)

[emilbayes]

If we use the long argument names we'll probably be good with cross compat. Otherwise good to merge

[christianbundy]

Thanks! I've updated my PR with a new commit, which also comes with readme output for cross-checking. I've updated the original post in this PR with the new commit message explaining the changes.