sys/Makefile.include: check for newlib_nano instead of USE_NANO_SPECS
sys/Makefile.include: check for newlib_nano instead of USE_NANO_SPECS

Check for the usage of `newlib_nano` module instead of the
`USE_NANO_SPECS` variable.

This allows also benefiting from the `printf_float` and `scanf_float`
behaviour on `arm7` and `riscv` cpus.

cladmi/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,smlng/RIOT,jasonatran/RIOT,cladmi/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,x3ro/RIOT,authmillenon/RIOT,basilfx/RIOT,mtausig/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,ant9000/RIOT,rfuentess/RIOT,josephnoir/RIOT,mtausig/RIOT,miri64/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,x3ro/RIOT,josephnoir/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,yogo1212/RIOT,aeneby/RIOT,smlng/RIOT,miri64/RIOT,smlng/RIOT,aeneby/RIOT,OlegHahm/RIOT,aeneby/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,kYc0o/RIOT,josephnoir/RIOT,ant9000/RIOT,yogo1212/RIOT,authmillenon/RIOT,toonst/RIOT,authmillenon/RIOT,x3ro/RIOT,kaspar030/RIOT,aeneby/RIOT,basilfx/RIOT,smlng/RIOT,rfuentess/RIOT,OTAkeys/RIOT,toonst/RIOT,x3ro/RIOT,cladmi/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,yogo1212/RIOT,aeneby/RIOT,mtausig/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,OTAkeys/RIOT,toonst/RIOT,OTAkeys/RIOT,cladmi/RIOT,OlegHahm/RIOT,cladmi/RIOT,toonst/RIOT,yogo1212/RIOT,OlegHahm/RIOT,smlng/RIOT,josephnoir/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,basilfx/RIOT,basilfx/RIOT,kaspar030/RIOT,x3ro/RIOT,mtausig/RIOT,mtausig/RIOT,rfuentess/RIOT