link-time-opt should not be using -fwhole-program

When building hotspot with link-time-opt, we are currently using -fwhole-program to the linker. This causes all public symbols to be removed. To counter this, the attribute "externally_visible" has been added to JNIEXPORT for arm (the platform that traditionally has used link-time-opt).

This is not an appropriate way of doing things.

The gcc documentation says about "-fwhole-program": "This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information."

If we replace -fwhole-program with -fuse-linker-plugin, the need to use externally_visible disappears.