Standardize of README.md for emscripten-specific readme files. NFC (emscripten-core#27296)

Author: sbc100

system/lib/compiler-rt/readme.txt system/lib/compiler-rt/README.mdsystem/lib/compiler-rt/readme.txt renamed to system/lib/compiler-rt/README.md

@@ -1,2 +1,3 @@
-Files in this directory are not strictly standard musl libc, but implemented here for compatibility for Emscripten purposes.
+Files in this directory are not strictly standard musl libc, but implemented
+here for compatibility for Emscripten purposes.
 

system/lib/libc/compat/readme.txt system/lib/libc/compat/README.mdsystem/lib/libc/compat/readme.txt renamed to system/lib/libc/compat/README.md

@@ -0,0 +1,23 @@
+llvm's OpenMP
+-----------
+
+These files are from the llvm-project based on release 21.1.8.
+
+We maintain a local fork of llvm-project that contains any Emscripten
+specific patches:
+
+  https://github.com/emscripten-core/llvm-project
+
+The current patch is based on the emscripten-libs-21 branch.
+
+Update Instructions
+-------------------
+
+Run `system/lib/update_llvm_openmp.py path/to/llvm-project`
+
+Modifications
+-------------
+
+For a list of changes from upstream see the OpenMP files that are part of:
+
+https://github.com/llvm/llvm-project/compare/llvmorg-21.1.8...emscripten-core:emscripten-libs-21

system/lib/libcxx/readme.txt system/lib/libcxx/README.mdsystem/lib/libcxx/readme.txt renamed to system/lib/libcxx/README.md

@@ -68,11 +68,9 @@ def get_llvm_version(upstream_dir):
 
 
 def update_readme(local_dir, llvm_dir):
-  readme_path = os.path.join(local_dir, 'readme.txt')
+  readme_path = os.path.join(local_dir, 'README.md')
   if not os.path.exists(readme_path):
-    readme_path = os.path.join(local_dir, 'README.txt')
-    if not os.path.exists(readme_path):
-      return
+    return
 
   with open(readme_path, 'r') as f:
     content = f.read()