Automatic detecting floating point printf usage

Current newlib-nano requires programers to explicity specify -u _printf_float to enable floating point IO, which is not practical in some cases like automatic project generation.

GCC should safely guess if floating point IO is needed, and then inform linker to pull-in necessary symbols in newlib-nano. The solution should be
- transparant (user needn't know this feature)
- compatible to existing projects (existing project with explicit -u _printf_float should just work)