It seems that the gcc toolchain on the RaspberryPi
likes %progbits instead of @progbits. I verified that
%progbits also works under x86, so this should
fix#2848 and fix#2842
I verified that unix and mpy-cross both compile
on my RaspberryPi and on my x64 machine.
This patch consolidates all global variables in py/ core into one place,
in a global structure. Root pointers are all located together to make
GC tracing easier and more efficient.
