To compile extensions, please use:
CFLAGS="-mtune=generic -Os -pipe"
CXXFLAGS="-mtune=generic -Os -pipe -fno-exceptions -fno-rtti"
Note that "-fno-exceptions -fno-rtti" will not work for all extensions - gtkmm and similar for example - but will result in smaller output when it does work.
..and you can try:
CC="gcc -flto -fuse-linker-plugin"
CXX="g++ -flto -fuse-linker-plugin"
..which doesn't always compile, but results in smaller output when it does work.
Apologies to the maintainers of x86 extensions whose *.info files were copied and pressed into service for the x86_64 repo. You are of course welcome to take over the maintenance of the relevant x86_64 extension if you so wish.
Note that x86 extensions that do not contain 32-bit binaries - font extensions for example - should work on x86_64.