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/calls/fstat-nt.c

@@ -71,7 +71,7 @@ static textwindows long GetSizeOfReparsePoint(int64_t fh) {
       }
     }
     if (x >= 0200) {
-      z += _bsrl(tpenc(x)) >> 3;
+      z += bsrl(tpenc(x)) >> 3;
     }
     ++z;
   }