Some of these things you are better off dealing with yourself. A few lines of code, stick it a header, and you are all set. No need for a library to do all this for you.
//Slp.h
#include <avr/io.h>
typedef enum { SLP_IDLE, SLP_STANDBY, SLP_PDOWN } slp_t;
static inline void slp_sleep(slp_t e){
SLPCTRL.CTRLA = (e<<1)|1;
sei();
asm("sleep");
SLPCTRL.CTRLA = 0;
}
Compare what you then get-
slp_sleep( SLP_PDOWN );
vs what libc will make you do, if you can figure out what all the macros are up to.