py/modsys: Add sys.implementation._build entry.

For a given MicroPython firmware/executable it can be sometimes important
to know how it was built, which variant/board configuration it came from.

This commit adds a new field `sys.implementation._build` that can help
identify the configuration that MicroPython was built with.

For now it's either:
* <VARIANT> for unix, webassembly and windows ports
* <BOARD>-<VARIANT> for microcontroller ports (the variant is optional)

In the future additional elements may be added to this string, separated by
a hyphen.

Resolves issue #16498.

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

tests/basics/sys1.py

@@ -24,6 +24,12 @@ else:
     # Effectively skip subtests
     print(int)
 
+if hasattr(sys.implementation, '_build'):
+    print(type(sys.implementation._build))
+else:
+    # Effectively skip subtests
+    print(str)
+
 try:
     print(sys.intern('micropython') == 'micropython')
     has_intern = True