Add configuration to build for cpu model

[DrDeano]

When building, add options for changing the CPU model.

const target = CrossTarget{
        .cpu_arch = .i386,
        .os_tag = .freestanding,
        .cpu_model = .{ .explicit = &Target.x86.cpu._i686 },
    };

Where changing .cpu_model = .{ .explicit = &Target.x86.cpu.<MODEL> },

With have to think about this and the best way of doing it without having hundards of options

[SamTebbs33]

Good idea. I reckon we could have a simple -Dcpu=cpu_name option and parse that to something we can use for cpu_model in Target.

For example, -Dcpu=skylake and -Dcpu=cortex_a72.

The CPU chosen would infer the architecture.

[DrDeano]

This can be done using b.standardTargetOptions()