games/stuntrally: fix build on PPC with clang and ARM

Upstream PR:
https://github.com/stuntrally/stuntrally/pull/18

We had until now CXXFLAGS_gcc=-Wno-narrowing, but it looks like this was incorrect because it did not fix the original issue.

PR: 242272
Approved by: linimon (mentor), amdmi3 (maintainer)