Reorder gnatmake arguments in inline-section-gc.exp

inline-section-gc.exp ends up passing "-lm" to gnatmake as an "marg"
-- meaning gnatmake should process it itself.  However, the gnat-llvm
gnatmake does not know what to do with this, so the test fails.

This patch rearranges the arguments so that the (implicit) trailing
-lm ends up being passed through to the linker.