Add musl update script. NFC.

I'm in the process of updateing musl using a git mirror that holds both
upstream musl and our local emscripten changes.  This scripte can be
used to copy new versions into place.

Author: sbc100

system/lib/update_musl.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+# Copyright 2020 The Emscripten Authors.  All rights reserved.
+# Emscripten is available under two separate licenses, the MIT license and the
+# University of Illinois/NCSA Open Source License.  Both these licenses can be
+# found in the LICENSE file.
+
+"""Simple script for updating musl from external git repo.
+
+The upstream sources, along with our local changes, live at:
+
+  https://github.com/emscripten-core/musl
+
+To update musl first make sure all changes from the emscripten repo
+are present in the `emscripten` branch or the above repo.  Then run
+`git merge v<musl_version>` to pull in the latest musl changes from
+a given musl version.  Once any merge conflict are resolved those
+change can then be copied back into emscripten using this script.
+"""
+
+import os
+import sys
+import shutil
+import subprocess
+
+script_dir = os.path.abspath(os.path.dirname(__file__))
+local_src = os.path.join(script_dir, 'libc', 'musl')
+exclude_dirs = (
+  # Top level directories we don't include
+  'tools', 'obj', 'lib', 'crt', 'musl', 'compat',
+  # Parts of src we don't build
+  'malloc',
+  # Arch-specific code we don't use
+  'arm', 'x32', 'sh', 'i386', 'x86_64', 'aarch64', 'riscv64',
+  's390x', 'mips', 'mips64', 'mipsn32', 'powerpc', 'powerpc64',
+  'm68k', 'microblaze', 'or1k', 'generic')
+
+
+musl_dir = os.path.abspath(sys.argv[1])
+
+
+def should_ignore(name):
+  return name in exclude_dirs or name[0] == '.'
+
+
+def ignore(dirname, contents):
+  return [c for c in contents if should_ignore(c)]
+
+
+def main():
+  assert os.path.exists(musl_dir)
+
+  # Remove old version
+  shutil.rmtree(local_src)
+
+  # Copy new version into place
+  shutil.copytree(musl_dir, local_src, ignore=ignore)
+
+
+if __name__ == '__main__':
+  main()