Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

third_party/compiler_rt/divti3.c

@@ -1,36 +1,22 @@
-/* clang-format off */
-/* ===-- divti3.c - Implement __divti3 -------------------------------------===
- *
- *                     The LLVM Compiler Infrastructure
- *
- * This file is dual licensed under the MIT and the University of Illinois Open
- * Source Licenses. See LICENSE.TXT for details.
- *
- * ===----------------------------------------------------------------------===
- *
- * This file implements __divti3 for the compiler_rt library.
- *
- * ===----------------------------------------------------------------------===
- */
-
-STATIC_YOINK("huge_compiler_rt_license");
-
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to division,     │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
 #include "third_party/compiler_rt/int_lib.h"
 
-#ifdef CRT_HAS_128BIT
-
-/* Returns: a / b */
-
-COMPILER_RT_ABI ti_int
-__divti3(ti_int a, ti_int b)
-{
-    const int bits_in_tword_m1 = (int)(sizeof(ti_int) * CHAR_BIT) - 1;
-    ti_int s_a = a >> bits_in_tword_m1;           /* s_a = a < 0 ? -1 : 0 */
-    ti_int s_b = b >> bits_in_tword_m1;           /* s_b = b < 0 ? -1 : 0 */
-    a = (a ^ s_a) - s_a;                         /* negate if s_a == -1 */
-    b = (b ^ s_b) - s_b;                         /* negate if s_b == -1 */
-    s_a ^= s_b;                                  /* sign of quotient */
-    return (__udivmodti4(a, b, (tu_int*)0) ^ s_a) - s_a;  /* negate if s_a == -1 */
+/**
+ * Divides 128-bit signed integers.
+ *
+ * @param a is numerator
+ * @param b is denominator
+ * @return quotient or result of division
+ * @note rounds towards zero
+ */
+COMPILER_RT_ABI ti_int __divti3(ti_int a, ti_int b) {
+  return __divmodti4(a, b, NULL);
 }
-
-#endif /* CRT_HAS_128BIT */