Fix 'test-make' build target in 11u

This is a tiny fix for a problem introduced by JDK-8212028 (which has already been downported to 11u) and fixed as part and fixed in jdk 12 as part of JDK-8210958 (which hasn't been downported yet and probably won't be donwported at all).

Instead of smuggling this fix into another downport, it has been argued (https://mail.openjdk.java.net/pipermail/jdk-updates-dev/2020-February/002533.html) that it is cleaner to create an explicit issue for fixing it in 11u.

The fix is trivial (that's why it was done as part of JDK-8210958 without an extra bug ID) and only affects running "make test-make":

diff -r 8599975f5b33 test/make/TestMakeBase.gmk
--- a/test/make/TestMakeBase.gmk Tue Feb 12 08:40:43 2019 +0100
+++ b/test/make/TestMakeBase.gmk Mon Dec 23 22:10:46 2019 +0100
 KWBASE := APA=banan;GURKA=tomat;COUNT=1%202%203%204%205;SUM=1+2+3+4+5;MANY_WORDS=I
have the best words.

 $(eval $(call ParseKeywordVariable, KWBASE, \
- KEYWORDS := APA GURKA SUM, \
+ SINGLE_KEYWORDS := APA GURKA SUM, \
     STRING_KEYWORDS := COUNT MANY_WORDS, \
 ))

@@ -364,7 +376,7 @@
 KWBASE_WEIRD := ;;APA=banan;;;GURKA=apelsin;APA=skansen;;

 $(eval $(call ParseKeywordVariable, KWBASE_WEIRD, \
- KEYWORDS := APA GURKA SUM, \
+ SINGLE_KEYWORDS := APA GURKA SUM, \
     STRING_KEYWORDS := COUNT, \
 ))