Add bit_set to core:sys/posix with all OS types that should be POSIX compliant

[RaphGL]

I've been writing a library and I needed to conditionally compile a function so if the OS was POSIX I could do a syscall otherwise provide a fallback.

With this bit_set I could do this:

when ODIN_OS in posix.ALL_VALID_OS_TYPES {
    my_func :: proc() { /* do something */ }
} else {
    my_func :: proc() { /* do something */ }
}

[laytan]

This should probably just be solved like we do in core:thread and core:time among others, where we have an IS_SUPPORTED constant that is true or false based on the OS. Your code would then be when posix.IS_SUPPORTED {