cmd/llgo-build: compile C output to workdir
Checked in the source tree for the boehm garbage collector. Projects ivmai/libatomic_ops and ivmai/bdwgc.
Added a proxy c-file including all the gc-functionality
Added pseudo-generic mechanism that finds directories named 'include' and adds them to the header-lookup-path for C-files compiled in packages
Added the Boehm GC to the llgo runtime. Added compiler-flag -gc to llgo-build that turns the GC on/off.