hardening: Default randstruct off with rust for better allmodconfig support

Currently randstruct does not support rust so we have Kconfig dependencies
which prevent rust being enabled when randstruct is. Unfortunately this
prevents rust being enabled in allmodconfig, our standard coverage build.
randstruct gets turned on by default, then the dependency on !RANDSTRUCT
causes rust to get disabled.

Work around this by disabling randstruct by default if we have a usable
rust toolchain and rust support for the architecture, circular
dependencies prevent us directly depending on !RUST. This means we might
end up with a configuration that disables both rust and randstruct but
hopefully it's more likely go give the expected result.

Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 6923036..81c81ad 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -278,7 +278,7 @@
 
 choice
 	prompt "Randomize layout of sensitive kernel structures"
-	default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
+	default RANDSTRUCT_FULL if !(RUST_IS_AVAILABLE && HAVE_RUST) && COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
 	default RANDSTRUCT_NONE
 	help
 	  If you enable this, the layouts of structures that are entirely