[cmake] Add SerenityOS to config.guess

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D154403

Author: ADKaster

llvm/cmake/config.guess

@@ -816,6 +816,9 @@ EOF
     i*:PW*:*)
 	echo ${UNAME_MACHINE}-pc-pw32
 	exit ;;
+    *:SerenityOS:*:*)
+	echo ${UNAME_MACHINE}-pc-serenity
+	exit ;;
     *:Interix*:*)
 	case ${UNAME_MACHINE} in
 	    x86)