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

libc/runtime/asan.greg.c

@@ -19,31 +19,57 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
 #include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
 #include "libc/sysv/consts/fileno.h"
 
+struct SourceLocation {
+  const char *filename;
+  int line;
+  int column;
+};
+
+struct AccessInfo {
+  const uint8_t *addr;
+  const uint8_t *first_bad_addr;
+  size_t size;
+  bool iswrite;
+  unsigned long ip;
+};
+
+struct Global {
+  const uint8_t *addr;
+  size_t size;
+  size_t size_with_redzone;
+  const void *name;
+  const void *module_name;
+  unsigned long has_cxx_init;
+  struct kasan_source_location *location;
+  char *odr_indicator;
+};
+
 privileged void __asan_init(void) {
 }
 
-privileged void __asan_register_globals(uintptr_t a, uintptr_t b) {
+privileged void __asan_version_mismatch_check_v8(void) {
 }
 
-privileged void __asan_unregister_globals(uintptr_t a, uintptr_t b) {
+privileged void __asan_register_globals(struct Global globals[], int n) {
+}
+
+privileged void __asan_unregister_globals(struct Global globals[], int n) {
+}
+
+privileged void __asan_report_load_n(uint8_t *p, int n) {
+}
+
+privileged void __asan_report_store_n(uint8_t *p, int n) {
+  __asan_report_load_n(p, n);
 }
 
 privileged void __asan_loadN(uintptr_t ptr, size_t size) {
-  dprintf(STDERR_FILENO, "load %p %zu");
 }
 
 privileged void __asan_storeN(uintptr_t ptr, size_t size) {
-  dprintf(STDERR_FILENO, "store %p %zu");
-}
-
-privileged void __asan_report_load_n(uintptr_t ptr, size_t size) {
-  dprintf(STDERR_FILENO, "%s%zu%s%#p", "asan: load ", size, " bytes at ", ptr);
-}
-
-privileged void __asan_report_store_n(uintptr_t ptr, size_t size) {
-  dprintf(STDERR_FILENO, "%s%zu%s%#p", "asan: store ", size, " bytes at ", ptr);
 }
 
 privileged uintptr_t __asan_stack_malloc(size_t size, int classid) {
@@ -54,9 +80,7 @@ privileged void __asan_stack_free(uintptr_t ptr, size_t size, int classid) {
 }
 
 privileged void __asan_handle_no_return(void) {
-}
-
-privileged void __asan_version_mismatch_check_v8(void) {
+  DebugBreak();
 }
 
 privileged void __asan_alloca_poison(uintptr_t addr, uintptr_t size) {