configure: abort on UBSan failure

By default, UBSan reports runtime errors but does not stop execution.
We already abort in debug builds, and this commit makes us also abort in
regular builds when UBSan is enabled. Arguably, this is what users
expect when they enable UBSan, so it is a good default.

I know I've missed some UBSan bugs in the past because of this issue,
and dergoegge mentioned that this also happened to him.

configure

@@ -165,10 +165,7 @@ set_defaults()
 	fi
     fi
     if [ "$UBSAN" != 0 ]; then
-	CSANFLAGS="$CSANFLAGS -fsanitize=undefined"
-	if [ "$DEBUGBUILD" != 0 ]; then
-	    CSANFLAGS="$CSANFLAGS -fno-sanitize-recover=undefined"
-	fi
+	CSANFLAGS="$CSANFLAGS -fsanitize=undefined -fno-sanitize-recover=undefined"
     fi
     if [ "$FUZZING" != 0 ]; then
 	FUZZFLAGS="-fsanitize=fuzzer-no-link -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"