Backport e2k support to mbedtls-2.7

Covers commits ac2fd65, 0be6aa9, e74f372, e559550
from `development` branch

Signed-off-by: makise-homura <akemi_homura@kurisa.ch>
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index 523db86..0c27243 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -244,7 +244,7 @@
 /* A compile-time constant with the value 0. If `const_expr` is not a
  * compile-time constant with a nonzero value, cause a compile-time error. */
 #define STATIC_ASSERT_EXPR( const_expr )                                \
-    ( 0 && sizeof( struct { int STATIC_ASSERT : 1 - 2 * ! ( const_expr ); } ) )
+    ( 0 && sizeof( struct { unsigned int STATIC_ASSERT : 1 - 2 * ! ( const_expr ); } ) )
 /* Return the scalar value `value` (possibly promoted). This is a compile-time
  * constant if `value` is. `condition` must be a compile-time constant.
  * If `condition` is false, arrange to cause a compile-time error. */