Generalize -msse2 command line flag to -msse<version number>

[tibbe]

I'm looking into adding support for the popcnt instruction. The instruction should be used if the user specifies -msse4.2*. I think we should generalize the -msse2 flag to be just -msse followed by a version number.

* Although not formally part of the SSE 4.2 instruction set, the popcnt instruction was added at the same time. GCC enables the popcnt instruction if you specify -msse4.2, so I thought it'd be reasonable for us to do so as well.