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.

libc/fmt/formatbinary64.c

@@ -45,7 +45,7 @@ char *FormatBinary64(char p[hasatleast 67], uint64_t x, char z) {
       *p++ = '0';
       *p++ = 'b';
     }
-    i = PickGoodWidth(_bsrl(x));
+    i = PickGoodWidth(bsrl(x));
     do {
       b = 1;
       b <<= i;