[AIX] Use pthreads to avoid JNI_createVM call from primordial thread

Created on behalf of Varada M

Currently several tests have `@require os.family != "aix"` because the tests call JNI_createVM. There are issues which prevent jvm creation on the primordial thread on AIX. However, these tests can be refactored to run in pthreads to enable them to test AIX as well.

The following tests can benefit from this change:
test/jdk/jni/nullCaller/NullCallerTest.java
test/jdk/java/lang/reflect/exeCallerAccessTest/CallerAccessTest.java
test/hotspot/jtreg/runtime/jni/CalleeSavedRegisters/FPRegs.java
test/hotspot/jtreg/runtime/jni/daemonDestroy/TestDaemonDestroy.java
test/lib-test/jdk/test/lib/process/TestNativeProcessBuilder.java