Testsuite: Skip failing tests on PowerPC 64-bit

The Power ISA says the result of a division by zero is undefined.  So
ignore stdout on PowerPC 64-bit systems.

Disable ext-interp tests on 64-bit PowerPC.  We don't have support for
PowerPC 64-bit ELF in the RTS linker, which is needed for the external
interpreter.

Test Plan: ./validate

Reviewers: austin, simonmar, hvr, erikd, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2782