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/intrin/bsr.c

@@ -23,7 +23,7 @@
  * Returns binary logarithm of 𝑥.
  *
  *                           ctz(𝑥)         31^clz(𝑥)   clz(𝑥)
- *       uint32 𝑥  _bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)  _bsr(𝑥) lzcnt(𝑥)
+ *       uint32 𝑥   bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)   bsr(𝑥) lzcnt(𝑥)
  *     0x00000000      wut       32        0      wut       32
  *     0x00000001        0        0        1        0       31
  *     0x80000001        0        0        1       31        0
@@ -36,7 +36,7 @@
  * @param x is a 32-bit integer
  * @return number in range 0..31 or undefined if 𝑥 is 0
  */
-int(_bsr)(int x) {
+int(bsr)(int x) {
   int r = 0;
   if(x & 0xFFFF0000u) { x >>= 16; r |= 16; }
   if(x & 0xFF00) { x >>= 8; r |= 8; }