[SDK] Add GCC overrides for SAL annotations

GCC has some functions, variables & type attributes which can be used as aliases
for some of the SAL annotations. Although it's not as rich & precise, it's still useful
since we actually enable -Werror on GCC builds whereas we don't use such an option
on MSVC builds.

For now, _Must_inspect_result_ is aliased to warn_result_unused attribute.

sdk/lib/rtl/CMakeLists.txt

@@ -5,6 +5,11 @@ add_definitions(
     -D_NTSYSTEM_
     -D_NTDLLBUILD_)
 
+if (GCC)
+    # Enable this again. CORE-17637
+    add_compile_options(-Wunused-result)
+endif()
+
 list(APPEND SOURCE
     access.c
     acl.c