games/stuntrally: fix build with GCC-based architectures

[pkubaj]

Summary

Add -Wno-narrowing to CXXFLAGS when using GCC to fix build error.

PR: 237777

[tcberner]

[mat]

This can be written in a really simpler way like this:

CXXFLAGS+=  ${CXXFLAGS_${CHOOSEN_COPMILER_TYPE}}

CXXFLAGS_gcc= -Wno-narrowing

And no need to do the pre/post dance.