plugins/jvm: let override jvm arch

On mixed 64 bit kernel / 32 bit userspace linux systems the jvm
does not get recognized automatically if installed.

Permit to force an arch by exporting JVM_ARCH which is way more
easier than exporting the full path for includes and libraries.
Merge pull request #441 from xrmx/jvmarch2

plugins/jvm: let override jvm arch