The linker flag --gc-sections is not available on the linker used on
Mac OS X which results in an error when linking micropython on Mac OS X.
Therefore move this option to the LDFLAGS_ARCH variable on non Darwin
systems. According to http://stackoverflow.com/a/17710056 the equivalent
to --gc-sections is -dead_strip thus this option is used for the
LDFLAGS_ARCH on Darwin systems.