feat: init x86_16 arch via CBE

[GasInfinity]

This adds support for x86_16, a.k.a IA-16, a.k.a i86 via CBE targeting the IA-16 fork of GCC.

Primarily targeted for freestanding and DOS programs (other). Support for far pointers should be done in a follow-up PR as it involves lowering them in CBE.

As usize is 16-bits I had to add some explicit @intCasts in compiler_rt, I'm sure a lot of std code will have the same issue (#10669).

Here's a snippet I tested in DOSBOX:

// zig build-exe -target x86_16-other main.zig -OReleaseFast -ofmt=c
// ia16-elf-gcc main.c -Os -o main.exe

export fn main(argc: usize, argv: [*]const [*:0]const c_char) callconv(.c) u8  {
    _ = argv;
    var fmt_buf: [1024]u8 = undefined;
    dosPrintString(std.fmt.bufPrintZ(&fmt_buf, "Hello DOS from zig using int 21H (AH = 02H, a.k.a character output)!\nGot {} arguments!\n", .{
        argc,
    }) catch unreachable);
    return 0;
}

fn dosPrintString(str: []const u8) void {
    for(str) |c| {
        asm volatile("int $0x21" :: [code] "{ah}" (@as(u8, 2)), [character] "{dl}" (@as(u8, c)) : .{
            .cc = true,
            .al = true,
        });
    }
}

const std = @import("std");

Partially addresses #7469

[alexrp]

In this case, the question is what compilers support, not what the architecture supports.

Which might actually be an argument for moving supportsAddressSpace to std.zig.target... but that's a problem for another day.

[alexrp]

I don't think we have precedent for addrspace affecting pointer size; we kind of just assume that all pointers have the same size. That's wrong for many reasons and will have to change one way or another, but just so you know, here be dragons.

[alexrp]

This looks good to me now.

Just to keep history clean, would you mind rewriting this into 4 commits:

• One that adjusts the intByteSize return type
• One that makes the adjustments needed for 16-bit usize (compiler-rt, std.Io.Writer, std.os.windows)
• One that adds 16-bit x86 support in zig.h
• One that adds all the target information, calling conventions, etc