Possible values for `uname -m`

The uname(1) command-line utility has a -m option which prints the "machine hardware name".

On Linux, this field comes from the machine member of struct utsname, as populated by the uname(2) system call.

Many other language APIs return this information:

• Python: os.uname() and platform.machine()
• PHP: php_uname()

What are the possible values for the "machine" field?