Update ARCH_FLAGs and some old 64-bit related comments.

The advantage is in more modern cycle costs for x86 instructions. Pentium 4 was notorious for extremely high costs of some rather simple instructions. For example (but not really affecting this), a NaN or infinity could push a simple instruction to 900 cycles.
By telling the compiler to tune to a more modern cpu, you get more realistic cycle costs used in determining the assembly output.