RFR(XS): 8066589: Make importing sa-jdi.jar optional on its existance

[Volker Simonis]

Hi,

could you please review this tiny change which fixes the build on
platforms which don't built the SA agent after the recent modualrity
integrations:

http://cr.openjdk.java.net/~simonis/webrevs/8066589
https://bugs.openjdk.java.net/browse/JDK-8066589

I've tested that the fix works on AIX but I havn't had a chance to build Zero.

@Xerxes: maybe you could check if my suggested fix also solves your
build problems. I'm also no sure if the "ifneq ($(JVM_VARIANT_ZERO),
true)" clause also catches the ZEROSHARK case (altough I think it
should). If not we would need yet another "ifneq
($(JVM_VARIANT_ZEROSHARK), true)"

Thanks,
Volker