avr: Add vfcanf implementation selection

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
macros: simplify for --std=gnu99

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
avr: ditch c99 warnings, add initial atomics support for avr

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>