libsel4test: retain _test_type and _test_case

unused attribute does not prevent the sections from being
garbage-collected during link-time optimisation. This may trigger
undefined references errors to [__start|__stop]_test_case symbols
that are expected to be emitted by the linker anyway.

Adding "retain" attribute makes sure that the section and its
associated symbols are kept regardless of linker's garbage
collection. Another fix could be adding "nostart-stop-gc" to the
linker flags, but since it is only one section (_test_case)
where its __start/__stop symbols are references, adding retain to
it makes more sense. This additional functionality requires
binutils version 2.36 or later.

Sponsored by: DARPA.

Signed-off-by: Hesham Almatary <hesham.almatary@cl.cam.ac.uk>

libsel4test/include/sel4test/test.h

@@ -96,10 +96,16 @@ typedef struct test_type {
     test_result_t (*run_test)(struct testcase *test, uintptr_t e);
 } ALIGN(32) test_type_t;
 
+#if defined(__has_attribute) && __has_attribute(retain)
+#define ATTR_USED_RETAIN __attribute__((used,retain))
+#else
+#define ATTR_USED_RETAIN __attribute__((used))
+#endif
+
 /* Declare a test type.
  * For now, we put the test types in a separate elf section. */
 #define DEFINE_TEST_TYPE(_name, _id, _set_up_test_type, _tear_down_test_type, _set_up, _tear_down, _run_test) \
-    __attribute__((used)) __attribute__((section("_test_type"))) struct test_type TEST_TYPE_ ##_name = { \
+    ATTR_USED_RETAIN __attribute__((section("_test_type"))) struct test_type TEST_TYPE_ ##_name = { \
     .name = #_name, \
     .id = _id, \
     .set_up_test_type = _set_up_test_type, \
@@ -134,7 +140,7 @@ typedef struct testcase ALIGN(sizeof(struct testcase)) testcase_t;
  * that it is accepted by C++ compilers.
  */
 #define DEFINE_TEST_WITH_TYPE(_name, _description, _function, _test_type, _enabled) \
-    __attribute__((used)) __attribute__((section("_test_case"))) struct testcase TEST_ ## _name = { \
+    ATTR_USED_RETAIN __attribute__((section("_test_case"))) struct testcase TEST_ ## _name = { \
     #_name, \
     _description, \
     (test_fn)_function, \