Added `std.system.instructionSetArchitecture` and `std.system.ISA`

A new enum representing the instruction set architecture for the targeted
system was added. It is intended for cases where a targeted CPU's ISA is only
needed at runtime, such as providing human-readable messages as demonstrated
below.
-------
import std.stdio;
import std.system;

void main()
{
    writeln("Hello ", instructionSetArchitecture, " world!");
}
-------
