Add meta function CPacked to add align(1) to extern structs

[fifty-six]

This allows for easier FFI with C APIs that use large amounts of packed structs by allowing the omission of align(1) on every field declaration.

Motivation behind this being for UEFI ffi as almost every struct there is packed.

[wooster0]

Sadly this isn't actually going to work if the extern struct has any decls inside it...

    const S3 = CPacked(extern struct {
        a: u8,
        b: u8,
        c: u16,
        d: u8,
        fn x() void {}
    }, .{ .b = @alignOf(u16) });

x.zig:149:12: error: reified structs must have no decls
    return @Type(.{
           ^~~~~
x.zig:168:23: note: called from here
    const S3 = CPacked(extern struct {
               ~~~~~~~^

Yet another reason for #6709

So I don't think this will work as a permanent solution. It would work as a better temporary one if #6709 was solved but ultimately the language needs an addition to make this easier.

[fifty-six]

if anyone is interested in this for just simple data I can reopen but without decls I'm not sure this is great and a lang feature would be better