Improve performance of bitscanning intrinsics

This change helps spectre more intelligently plan execution, by working
around false output dependencies, impacting ops like popcnt bsr and bsf

libc/testlib/blocktronics.S

@@ -0,0 +1,33 @@
+/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8     -*-│
+│vi: set et ft=asm ts=8 tw=8 fenc=utf-8                                     :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/macros.internal.h"
+.source	__FILE__
+.rodata
+
+//	Nontrivial NUL-terminated string test vector.
+	.align	1
+kBlocktronics:
+0:	.incbin	"libc/testlib/blocktronics.txt"
+1:	.byte	0
+	.endobj	kBlocktronics,globl
+
+	.align	8
+kBlocktronicsSize:
+	.quad	1b-0b
+	.endobj	kBlocktronicsSize,globl