> ahhh.. It's partly the assignment as Matt said and also -ffreestanding.
   
   Oh, don't we use __builtin_strlen() explicitly (see libkern.h)?

yup:

libkern.h:#define       memcpy(d, s, l)         __builtin_memcpy(d, s, l)
libkern.h:#define       memcmp(a, b, l)         __builtin_memcmp(a, b, l)
libkern.h:#define       memset(d, v, l)         __builtin_memset(d, v, l)
libkern.h:#define       strcpy(d, s)            __builtin_strcpy(d, s)
libkern.h:#define       strcmp(a, b)            __builtin_strcmp(a, b)
libkern.h:#define       strlen(a)               __builtin_strlen(a)
libkern.h:#define       alloca(s)               __builtin_alloca(s)
libkern.h:#define       ffs(x)                  __builtin_ffs(x)

for a complete list.