[TESTBUG] jfr/event/sampling/TestNative was backported to JDK8u without proper native wrapper

jfr/event/sampling/TestNative arrived to JDK8u with whole JFR backport. However JDK8 do not have helper functions for jtregs with native code. A shell wrapper is needed to make this test passing.
On most of the combinations, this is the only failing test in jdk_jfr group.

Exemplar fix is provided here.