Fix build of java/bouncycastle15 on armv6:
remove fork="true" in javac and fork="yes" in junit tasks (ant/bc+-build.xml)

Original report:

If java/bouncycastle15 is build on armv6 with openjdk18, the build stops with
This command is not for general use and should only be run as the result of a call to ProcessBuilder.start() or Runtime.exec() in a java application

PR: 220612
Submitted by: Gerrit Beine <mail+freebsd@gerritbeine.de> (based on)
Approved by: az (mentor)