Compile from source on linux arch64

[g-poveda]

Somehow related to #80 ,
I didn't manage to pip install pynng on manylinux docker instance running on arm64.
Therefore i tried installing from source, though i had the same issue at the end the flag "-latomic" was an issue (even though i had it installed) and made the compil buggued.

So when looking at this piece of code, i found a workaround

pynng/build_pynng.py

Line 40 in 9b651be

# this is a pretty heuristic... but let's go with it anyway.

by not adding the "atomic" library also in my corner case (linux arch64), and it worked.

From this piece of code i highlight, it may be worth to consider the case of linuxArch64 in the "if" conditions, if my problem occurs on all linuxarch64 instance. It may make the direct pip install pynng working too.