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/python/Parser/node.c

@@ -31,7 +31,7 @@ fancy_roundup(int x)
     /* Round up to the closest power of 2 >= n. */
     int r;
     assert(x > 128);
-    r = 1u << (_bsr(x - 1) + 1); /* hacker's delight */
+    r = 1u << (bsr(x - 1) + 1); /* hacker's delight */
     return r > 0 ? r : -1;
 }