Changes in platform detection for buildout.

The multiarch strings (GNU target triplets) used currently for platform
detection are different in Suse, as they include the "vendor" ("suse" or
"pc" depending on the GCC version) and the OS part is just "linux"
instead of "linux-gnu".

This changes platform detection to use the constants available in
buildout for detecting the operating system and the `platform` module
for detecting the machine, instead of these multiarch strings.

See merge request nexedi/slapos!1648