Clarify compiler versions for __fallthrough__ attribute and feature t…

…esting via __has_attribute

Signed-off-by: Thomas Nyman <thomas.nyman@ericsson.com>

docs/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.md

@@ -352,6 +352,8 @@ The C17 standard[^C2017] does not provide a mechanism to mark intentional fallth
 #endif
 ~~~
 
+The `__fallthrough__` attribute is supported since GCC 7.0.0[^gcc-release-notes-7] and Clang 4.0.0[^clang-fallthrough]. Feature testing via `__has_attribute` is supported since GCC 5.0.0[^gcc-release-notes-5] and Clang 2.9.
+
 [^Polacek17]: Polacek, Marek, ["-Wimplicit-fallthrough in GCC 7"](https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7), Red Hat Developer, 2017-03-10
 
 [^Corbet19]: Corbet, Jonathan.  ["An end to implicit fall-throughs in the kernel"](https://lwn.net/Articles/794944/), LWN, 2019-08-01.
@@ -362,6 +364,12 @@ The C17 standard[^C2017] does not provide a mechanism to mark intentional fallth
 
 [^Howlett23]: Howlett, Liam,[tools: Rename __fallthrough to fallthrough](https://github.com/torvalds/linux/commit/f7a858bffcddaaf70c71b6b656e7cc21b6107cec), Linux Kernel Source, 2023-04-07.
 
+[^gcc-release-notes-5]: GCC team, [GCC 5 Release Series Changes, New Features, and Fixes](https://gcc.gnu.org/gcc-5/changes.html), 2017-10-10.
+
+[^gcc-release-notes-7]: GCC team, [GCC 7 Release Series Changes, New Features, and Fixes](https://gcc.gnu.org/gcc-7/changes.html), 2019-11-14.
+
+[^clang-fallthrough]: LLVM team, [Attributes in Clang: fallthrough, clang::fallthrough](https://releases.llvm.org/4.0.0/tools/clang/docs/AttributeReference.html#fallthrough-clang-fallthrough), Clang Documentation, 2017-03-13.
+
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 ### Enable warnings for possibly misleading Unicode bidirectional control characters