Fix GCC 14 -Wstringop-overflow warnings on ppc64le

[runlevel5]

Summary

• neo/idlib/Swap.h: Use C++17 if constexpr in idSwap::Big() so the compiler discards dead branches at compile time, preventing false positive -Wstringop-overflow warnings when GCC inlines and analyzes out-of-bounds accesses in branches that can never execute (e.g. the sizeof(type) == 8 branch for a 4-byte float). Also replaced the runtime assert(false) fallback with static_assert for a compile-time error on unsupported type sizes.
• neo/d3xp/gamesys/SysCmds.cpp: Cast idStr::snPrintf() return value to unsigned int in PrintFloat() to tell the compiler the value cannot be negative (buf is 128 bytes, format string is fixed), preventing a false positive -Wstringop-overflow warning when GCC inlines PrintFloat into Cmd_ListDebugLines_f and considers a negative index path.