You can access the newlib source in a few ways:

Read-only web-based GIT access

You can use the gitweb interface.

Anonymous GIT read-only access
  git clone https://sourceware.org/git/newlib-cygwin.git
GIT for developers with read/write access
  git clone sourceware.org:/git/newlib-cygwin.git
Source snapshots

Source snapshots are available as gzipped tar files via anonymous FTP from the newlib ftp directory or http. New snapshots are packaged and released from time to time. Documentation snapshots are also provided.

Read-only web-based GIT access to newlib website