Rename _bsr/_bsf to bsr/bsf

Now that these functions are behind _COSMO_SOURCE there's no reason for
having the ugly underscore anymore. To use these functions, you need to
pass -mcosmo to cosmocc.

third_party/chibicc/tokenize.c

@@ -680,7 +680,7 @@ static void convert_universal_chars(char *p) {
           p += 16;
           q += 16;
         } else {
-          m = _bsf(m);
+          m = bsf(m);
           memmove(q, p, m);
           p += m;
           q += m;