Default to turning on architecture specific optimizations in the codegen
LLVM defaults to turning on whatever architecture specific optimizations are supported on the user's machine (e.g.
-msse4.2). This makes programs run faster during testing and generally gives users a better perception of the compiler. If the user wants to compile for some more portable subset (e.g. when cross-compiling) he/she adds
-mcpu, etc flags on the command line.
To implement this we'd have to detect what CPU type is used etc and initialize our
DynFlags based on that.