Use python3 when building on non-Windows for Windows

The makefiles look for python3 on Unix-like systems where python is often
Python 2. This uses sh code so it doesn't work on Windows. On Windows, the
makefiles just assume that python is Python 3.

The code was incorrectly deciding not to try python3 based on WINDOWS_BUILD,
which indicates that the build is *for* Windows. Switch to checking WINDOWS,
which indicates that the build is *on* Windows.

Fix Mbed-TLS#4774

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/makefile-python-windows.txt

@@ -0,0 +1,4 @@
+Bugfix
+   * The GNU makefiles invoke python3 in preference to python except on Windows.
+     The check was accidentally not performed when cross-compiling for Windows
+     on Linux. Fix this. Fixes #4774.

tests/Makefile

@@ -49,11 +49,15 @@ LOCAL_LDFLAGS += -lws2_32
 ifdef SHARED
 SHARED_SUFFIX=.$(DLEXT)
 endif
-PYTHON ?= python
 else
 DLEXT ?= so
 EXEXT=
 SHARED_SUFFIX=
+endif
+
+ifdef WINDOWS
+PYTHON ?= python
+else
 PYTHON ?= $(shell if type python3 >/dev/null 2>/dev/null; then echo python3; else echo python; fi)
 endif