Make build hermetic without shell scripts

- Fix some minor issues in ar.com
- Have execve() look for `ape` command
- Rewrite NT paths using /c/ rather /??/c:/
- Replace broken GCC symlinks with .sym files
- Rewrite $PATH environment variables on startup
- Make $(APE_NO_MODIFY_SELF) the default bootloader
- Add all build command dependencies to build/bootstrap
- Get the repository mostly building from source on non-Linux

tool/build/mkdir.c

@@ -0,0 +1,82 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to this file,    │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include "libc/calls/calls.h"
+#include "libc/errno.h"
+#include "libc/fmt/conv.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/ex.h"
+#include "libc/x/x.h"
+#include "third_party/getopt/getopt.h"
+
+#define USAGE \
+  " [-p] [-m MODE] DIR...\n\
+Utility for creating directories.\n\
+\n\
+FLAGS\n\
+\n\
+  -h       Help\n\
+  -m MODE  Octal mode\n\
+  -p       Make parent directories\n"
+
+const char *prog;
+
+wontreturn void PrintUsage(int rc, FILE *f) {
+  fputs("Usage: ", f);
+  fputs(prog, f);
+  fputs(USAGE, f);
+  exit(rc);
+}
+
+int main(int argc, char *argv[]) {
+  int i, mode = 0755;
+  int (*mkdirp)(const char *, unsigned) = mkdir;
+  prog = argc > 0 ? argv[0] : "mkdir.com";
+
+  while ((i = getopt(argc, argv, "?hpm:")) != -1) {
+    switch (i) {
+      case 'p':
+        mkdirp = makedirs;
+        break;
+      case 'm':
+        mode = strtol(optarg, 0, 8);
+        break;
+      case '?':
+      case 'h':
+        PrintUsage(0, stdout);
+      default:
+        PrintUsage(EX_USAGE, stderr);
+    }
+  }
+
+  if (optind == argc) {
+    fputs(prog, stderr);
+    fputs(": missing argument\n", stderr);
+    fputs("Try '", stderr);
+    fputs(prog, stderr);
+    fputs(" -h' for more information.\n", stderr);
+    exit(1);
+  }
+
+  for (i = optind; i < argc; ++i) {
+    if (mkdirp(argv[i], mode) == -1) {
+      fputs(prog, stderr);
+      fputs(": cannot create directory '", stderr);
+      fputs(argv[i], stderr);
+      fputs("' ", stderr);
+      fputs(strerdoc(errno), stderr);
+      fputc('\n', stderr);
+      exit(1);
+    }
+  }
+
+  return 0;
+}