Fix issues preventing build with g++

[fwsGonzo]

Building with local g++ (multilib) should be straight-forward, however it doesn't work because there are many errors detected by g++ but not clang++.

Fix these problems one by one, by configuring with CXX=g++.

[fwsGonzo]

I have successfully built an image with g++ now, but it doesn't produce the .multiboot section. If i build the OS with clang and the service with g++, it will work because the multiboot section exists.

terminate called after throwing an instance of 'Elf_exception' what(): No section header named .multiboot 
CMakeFiles/prepend_bootloader.dir/build.make:57: recipe for target 'CMakeFiles/prepend_bootloader' failed

[fwsGonzo]

Initial support here: #1273

[fwsGonzo]

And, working 32-bit support here: #1431
Also compiles 64-bit properly now, but doesn't boot. Closing this as the support is good enough for its purpose, which is just compiling on userspace linux.