Write tests for new APE loader and fix bugs

- Add FreeBSD-specific mmap() flags
- Reduce size of the APE loader from 8kb to 4kb
- Work towards fixing the Makefile build on WSL
- Automate testing of APE no-modify-self behaviors
- Make the ape.S shell script code cleaner and tinier
- Improve the APE sanity check to test behavior better
- Fixed issue with ShowCrashReports() sigaltstack() on BSDs
- Delete symbols for S_MODE magnums which wasted compile time

If you checked out yesterday's APE commit, please run:

    rm -f /usr/bin/ape o/tmp/ape /tmp/ape "${TMPDIR:-/tmp}/ape"

Because this change fixes certain aspects of the new ABI. We don't have
automated migrations for APE loader versions yet. Thanks! You can also
download prebuilt binaries here:

- https://justine.lol/ape.elf    (Linux/FreeBSD/NetBSD/OpenBSD)
- https://justine.lol/ape.macho  (Apple)

Install the appropriate one as `/usr/bin/ape`.

libc/runtime/getinterpreterexecutablename.c

@@ -51,6 +51,9 @@ char *GetInterpreterExecutableName(char *p, size_t n) {
   if (n < 2) {
     errno = ENAMETOOLONG;
   } else if (IsWindows() || IsXnu()) {
+    // TODO(jart): Does XNU guarantee argv[0] is legit?
+    //             Otherwise we should return NULL.
+    //             What about OpenBSD?
     if (strlen(GetProgramExecutableName()) < n) {
       strcpy(p, GetProgramExecutableName());
       return p;