CMake: Review external suffixes to add architecture as well as platform

[weefuzzy]

See #40 (comment)

Apparently best practice is to have something like .pd_linux.l_ia64 to help disambiguate different architectures. Need to find out is this applies to all OSes, and what the magic strings are.

[weefuzzy]

The gospel seems to be

https://github.com/pure-data/pure-data/blob/18c9695ba4de543c51c2c013b45b99b8f1c35a19/src/s_loader.c#L32-L38

According to which .l_ia64 is accepted but wrong and would actually be .l_amd64 for linux, .d_amd64 on mac and so forth.