jdk-15+32 can't be built using OpenJDK 14

[Matthias Klose]

Seen with jdk-15+32 on platforms with just the zero VM.  The build fails with

Executing: [/usr/lib/jvm/java-14-openjdk-x32/bin/java -XX:+UseSerialGC -Xms32M
-Xmx512M -XX:TieredStopAtLevel=1 -Duser.language=en -Duser.country=US -cp
/<<PKGBUILDDIR>>/build/hotspot/variant-zero/tools/jvmti jvmtiGen -IN
/<<PKGBUILDDIR>>/src/hotspot/share/prims/jvmti.xml -XSL
/<<PKGBUILDDIR>>/src/hotspot/share/prims/jvmtiH.xsl -OUT
/<<PKGBUILDDIR>>/build/hotspot/variant-zero/gensrc/jvmtifiles/jvmti.h -PARAM
majorversion 15]
/<<PKGBUILDDIR>>/src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java:4252:
error: switch expressions are a preview feature and are disabled by default.
            switch (next.kind) {
            ^
  (use --enable-preview to enable switch expressions)
/<<PKGBUILDDIR>>/src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java:4253:
error: multiple case labels are a preview feature and are disabled by default.
                case MONKEYS_AT, ABSTRACT, FINAL, STRICTFP, CLASS, INTERFACE,
ENUM -> true;
                               ^
  (use --enable-preview to enable multiple case labels)
/<<PKGBUILDDIR>>/src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java:4253:
error: switch rules are a preview feature and are disabled by default.
                case MONKEYS_AT, ABSTRACT, FINAL, STRICTFP, CLASS, INTERFACE,
ENUM -> true;
                                                                                   ^
  (use --enable-preview to enable switch rules)


This is the first build in a bootcycle configuration.

Matthias