[llvm-libc] Update readme.txt (#24770)

We don't update libc from
https://github.com/emscripten-core/llvm-project 's `emscripten-libs-**`
branch anymore.

Also this wraps a few lines to 80 cols.

Author: aheejin

system/lib/llvm-libc/readme.txt

@@ -3,34 +3,21 @@ llvm's libc
 
 These files are from llvm-project's HEAD.
 
-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-20 branch.
-
-Currently in producgtion, we do not use LLVM's libc directly yet, but libcxx uses a subset 
-of headers from libc. So libcxx directly depends on the following directories:
+Currently in production, we do not use LLVM's libc directly yet, but libcxx uses
+a subset of headers from libc. So libcxx directly depends on the following
+directories:
 - libc/hdr
 - libc/include/llvm-libc-macros
 - libc/include/llvm-libc-types
 - libc/shared
 - libc/src/__support
 
-In addition, we are experimenting with using llvm-libc in overlay mode. You can enable this
-by running with `EMCC_CFLAGS=-lllvmlibc`. See https://libc.llvm.org/overlay_mode.html for more
-details.
+In addition, we are experimenting with using llvm-libc in overlay mode. You can
+enable this by running with `EMCC_CFLAGS=-lllvmlibc`. See
+https://libc.llvm.org/overlay_mode.html for more details.
 
 Update Instructions
 -------------------
 
 Run `system/lib/update_libcxx.py path/to/llvm-project`
 Run `system/lib/update_llvm_libc.py path/to/llvm-project`
-
-Modifications
--------------
-
-For a list of changes from upstream see the libc files that are part of:
-
-https://github.com/llvm/llvm-project/compare/llvmorg-20.1.4...emscripten-core:emscripten-libs-20