math/octave-forge-interval: fix build on powerpc64 elfv2

Clang 10 exits with following error:
Assertion failed: (!IsStrict && "Don't know how to expand for strict nodes."), function ExpandNode, file /usr/local/poudriere/jails/head-powerpc64/usr/src/contrib/llvm-project/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp, line 3638.

Use GCC on powerpc64 for the time being.