[MINOR] Avoid passing the PermGenSize option to IBM JVMs.

IBM's Java VM doesn't have the concept of a permgen, so this option shouldn't be passed when the vendor property shows it is an IBM JDK.

Author: Tim Ellison <t.p.ellison@gmail.com>
Author: Tim Ellison <tellison@users.noreply.github.com>

Closes #6055 from tellison/MaxPermSize and squashes the following commits:

3a0fb66 [Tim Ellison] Convert tabs back to spaces
6ad4266 [Tim Ellison] Remove unnecessary else clauses to reduce nesting.
d27174b [Tim Ellison] Merge branch 'master' of https://github.com/apache/spark into MaxPermSize
42a8c3f [Tim Ellison] [MINOR] Avoid passing the PermGenSize option to IBM JVMs.