kconfig/randconfig: enable UBSAN for randconfig

Introduce an additional Kconfig check to only offer the option if the
compiler supports -fsanitize=undefined.

We no longer use Travis CI, so the original motivation for not enabling
UBSAN might no longer present.  Regardless, the option won't be present in
the first place if the compiler doesn't support -fsanitize=undefined.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/Kconfig

@@ -37,6 +37,10 @@ config CC_HAS_VISIBILITY_ATTRIBUTE
 config CC_SPLIT_SECTIONS
 	bool
 
+# Compiler supports -fsanitize=undefined
+config CC_HAS_UBSAN
+	def_bool $(cc-option,-fsanitize=undefined)
+
 # Set code alignment.
 #
 # Allow setting on a boolean basis, and then convert such selection to an

xen/Kconfig.debug

@@ -98,7 +98,7 @@ config SCRUB_DEBUG
 
 config UBSAN
 	bool "Undefined behaviour sanitizer"
-	depends on HAS_UBSAN
+	depends on HAS_UBSAN && CC_HAS_UBSAN
 	help
 	  Enable undefined behaviour sanitizer. It uses compiler to insert code
 	  snippets so that undefined behaviours in C are detected during runtime.

xen/tools/kconfig/allrandom.config

@@ -1,4 +1,3 @@
 # Explicit option choices not subject to regular RANDCONFIG
 
 CONFIG_GCOV_FORMAT_AUTODETECT=y
-CONFIG_UBSAN=n