Made C's sizeof operator work with initialized void variables.

Added regression tests that check cc65's handling of void variables.

test/err/void-size2.c

@@ -0,0 +1,11 @@
+/*
+  !!DESCRIPTION!! Size of void cast
+  !!ORIGIN!!      cc65 regression tests
+  !!LICENCE!!     Public Domain
+  !!AUTHOR!!      Greg King
+*/
+
+unsigned test (void)
+{
+    return sizeof ((void)12345);
+}