Restore the UNAME_M fix which was accidently reverted in the gnome3 import.
This fixes the build on architechtures where MACHINE and MACHINE_ARCH to
differ. But put it in the -common port so it shared between both versions
of py-gobject3. [1]

Work around python plist issue [2]

PR: 188856 [1], 195342 [2], 195738 [2], 195748 [2]
Sumbitted by: sbruno [1], mva[2]