#869 Need to clarify availability of libgcc functions like __moddi3

[dkl]

fbc uses libgcc functions like __moddi3 for some operations. For example, __moddi3 doesn't seem to be available in x86_64 libgcc. Maybe these functions need to be marked with FB_RTL_OPT_32BIT to ensure fbc will only use them on 32bit.