Make improvements

- You can now run `make -j8 toolchain` on Windows
- You can now run `make -j` on MacOS ARM64 and BSD OSes
- You can now use our Emacs dev environment on MacOS/Windows
- Fix bug where the x16 register was being corrupted by --ftrace
- The programs under build/bootstrap/ are updated as fat binaries
- The Makefile now explains how to download cosmocc-0.0.12 toolchain
- The build scripts under bin/ now support "cosmo" branded toolchains
- stat() now goes faster on Windows (shaves 100ms off `make` latency)
- Code cleanup and added review on the Windows signal checking code
- posix_spawnattr_setrlimit() now works around MacOS ARM64 bugs
- Landlock Make now favors posix_spawn() on non-Linux/OpenBSD
- posix_spawn() now has better --strace logging on Windows
- fstatat() can now avoid EACCES in more cases on Windows
- fchmod() can now change the readonly bit on Windows

third_party/make/findprog.h

@@ -41,6 +41,10 @@ extern const char *find_in_path (const char *progname);
    directory.  A null PATH is equivalent to an empty PATH, that is, to the
    singleton list that contains only the current directory.
 
+   If DIRECTORY is not NULL, all relative filenames (i.e. PROGNAME when it
+   contains a slash, and the PATH elements) are considered relative to
+   DIRECTORY instead of relative to the current directory of this process.
+
    Determines the pathname that would be called by execlp/execvp of PROGNAME.
    - If successful, it returns a pathname containing a slash (either absolute
      or relative to the current directory).  The returned string can be used
@@ -52,6 +56,7 @@ extern const char *find_in_path (const char *progname);
        - EACCES: means that the program's file cannot be accessed (due to some
          issue with one of the ancestor directories) or lacks the execute
          permissions.
+       - ENOMEM: means out of memory.
    If OPTIMIZE_FOR_EXEC is true, the function saves some work, under the
    assumption that the resulting pathname will not be accessed directly,
    only through execl/execv or execlp/execvp.
@@ -60,6 +65,7 @@ extern const char *find_in_path (const char *progname);
      - On POSIX systems excluding Cygwin: a '/',
      - On Windows, OS/2, DOS platforms: a '/' or '\'. */
 extern const char *find_in_given_path (const char *progname, const char *path,
+                                       const char *directory,
                                        bool optimize_for_exec);