py/modsys: Introduce sys.implementation._machine constant.

This contains a string useful for identifying the underlying machine.  This
string is kept consistent with the second part of the REPL banner via the
new config option MICROPY_BANNER_MACHINE.

This makes os.uname() more or less redundant, as all the information in
os.uname() is now available in the sys module.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/sys.rst

@@ -70,6 +70,7 @@ Constants
 
    * *name* - string "micropython"
    * *version* - tuple (major, minor, micro), e.g. (1, 7, 0)
+   * *_machine* - string describing the underlying machine
    * *_mpy* - supported mpy file-format version (optional attribute)
 
    This object is the recommended way to distinguish MicroPython from other