goarch: map target os to windows for mingw* TARGET_OS

mingw32/mingw64 is not a supported value for GOOS, so map from 'mingw*' to
'windows' to enable building for windows targets.

(From OE-Core rev: 099bd78ce80772c42887104a40aa0a92fdaa6965)

Signed-off-by: Peter Morrow <pemorrow@linux.microsoft.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/goarch.bbclass

@@ -114,6 +114,8 @@ def go_map_mips(a, f, d):
 def go_map_os(o, d):
     if o.startswith('linux'):
         return 'linux'
+    elif o.startswith('mingw'):
+        return 'windows'
     return o