Support stack and global blocks

[mjbshaw]

There are five objectives here:

1. Make it so methods take the same number of parameters between Objective-C and Zig code. It's not a huge deal but it marginally simplifies parameter matching.
2. Make it easy to create block literals and consume them/invoke them.
3. Make it so users can manage resources with their blocks by providing copy/release functions.
4. Support global blocks, which have no-op copy/release implementations.
5. Reduce the number of levels of indirection and register shuffling with each invocation. This was done by making the invocation function take the block literal as a parameter instead of the previous implementation that used a dedicated helper to hide it.

General usage is something kinda like this:

const Context = extern struct {
    foo: i32,
    bar = f32,
};

const ctx = Context { .foo = 42, bar = 13.0 };
var block_literal = ns.stackBlockLiteral(function, ctx, null, null); // No resources to free, so no copy/release functions.
someFunctionThatConsumesBlocks(block_literal.asBlock());

// Can pass more parameters if block takes args, or return a value too.
fn function(block:  *ns.BlockLiteral(Context)) callconv(.C) void {
  std.debug.print("foo = {}, bar = {}\n", .{block.context.foo, block.context.bar});
}

There are two mildly annoying things here:

1. The function must be callconv(.C). There's no way to avoid this without introducing additional indirection.
2. The block's context must be compatible with the C ABI (e.g., it can't be a tuple). We could work around this by making the context an (aligned) array of bytes and then provide a context() helper method that just returns @as(*ContextType, @ptrCast(&block.context_bytes)).

Note that this PR will require a corresponding update to the main Mach repo to consume the new block-related APIs.

Also note that I don't intend on publicly exposing the libSystem bindings. It's fine to do it, though. They're just low-level enough that they're not enjoyable to work with so wrapper APIs are better IMO.

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[slimsag]

This looks like a great improvement!