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/getsymboltable.c

@@ -96,7 +96,13 @@ static struct SymbolTable *GetSymbolTableFromZip(struct Zipos *zipos) {
  * @note This code can't depend on dlmalloc()
  */
 static struct SymbolTable *GetSymbolTableFromElf(void) {
-  return OpenSymbolTable(FindDebugBinary());
+  int e;
+  const char *s;
+  if ((s = FindDebugBinary())) {
+    return OpenSymbolTable(s);
+  } else {
+    return 0;
+  }
 }
 
 /**
@@ -117,7 +123,7 @@ static struct SymbolTable *GetSymbolTableFromElf(void) {
  * Function tracing is disabled throughout the duration of this call.
  * Backtraces and other core runtime functionality depend on this.
  *
- * @return symbol table, or NULL w/ errno on first call
+ * @return symbol table, or NULL if not found
  */
 struct SymbolTable *GetSymbolTable(void) {
   struct Zipos *z;