fabs is missing from the libm implementation:

#include <math.h>

float use_fabsf (float x)
{
    return fabsf (x);
}

double use_fabs (double x)
{
    return fabs (x);
}

int main (void) { return 0; }

• Compiling with -ffreestanding or with -fno-builtin will run against a linker error +verbatim+ use_fabs.o: In function `use_fabsf':

use_fabs.c:(.text+0x24): undefined reference to `fabs' -verbatim-

 __ATTR_CONST__ static inline int isfinite (double __x)
                                          ^~~
/srv/local/gnu/install/gcc-6/avr/include/math.h:359:42: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'double'
 __ATTR_CONST__ static inline double copysign (double __x, double __y)

"inline" should be replaced by +verbatim+__inline__-verbatim-.