TypedPython: narrow GCC fastmath to reassociation + FMA contract, like clang and MSVC #368

Owner decision (via leader 2026-11-05). cgen's GCC TP_FASTMATH_BEGIN uses optimize("fast-math"), which also enables finite-math-only (isnan / x != x folded away), unlike the clang (reassociate + contract) and MSVC macros.

Change: GCC uses optimize("associative-math,no-signed-zeros,no-trapping-math,fp-contract=fast") only. SPEC N-8 states the three compilers allow the same scope.

Done when: a test checks that x != x / isnan inside a fastmath function still works (runs under GCC on Linux CI) and the generated GCC macro no longer contains fast-math.