Cleanup native systemtest makefile

[Mesbah-Alam]

• Remove dependency of openj9-systemtest native makefiles on PLATFORM variable
• Use uname commands to figure out platforms and platforms specific compiler settings
• Reduce unnecessary platform-specific compiler flags

Related to : https://github.com/AdoptOpenJDK/openjdk-systemtest/issues/413

Signed-off-by: Mesbah_Alam@ca.ibm.com Mesbah_Alam@ca.ibm.com

[Mesbah-Alam]

For extended testing of this change, please see https://github.com/AdoptOpenJDK/openjdk-systemtest/issues/413

[Mesbah-Alam]

This PR should be delivered with https://github.com/AdoptOpenJDK/stf/pull/106

[Mesbah-Alam]

@smlambert @llxia - please review

[llxia]

Please update the copyright. Thanks

[Mesbah-Alam]

Please update the copyright. Thanks

Done!