Link libgcc.a

This is needed on ARM. Without it, you get:

_build/main.native.o: In function `.L104':
camlinternalOO.o:(.text+0xb36c): undefined reference to `__aeabi_dcmpge'
camlinternalOO.o:(.text+0xb3a0): undefined reference to `__aeabi_dadd'
...