Remove -march argument on x86_64 build option.

GCC doesn't support pentium4 as a arch target for x86_64. It complains that the architecture doesn't support x86_64
Which to be fair the first few models didn't support 64bit.
Just remove the architecture setting since there isn't a need for it.
Merge pull request #108 from Sonicadvance1/wrong_arch

Remove -march argument on x86_64 build option.