Make it possible to opt in to PInvoke wrapper generation

[rolfbjarne]

If exception marshaling is enabled, we redirect calls to the objc_msgSend family of functions to our own wrappers that sets up exception handling for Objective-C. Since these handlers are generic (one per objc_msgSend function), they have to do significant work to calculate the stack size for the function call.

We can generate specialized wrapper functions instead, but it's currently hardcoded for a few specific scenarios (i386 with exception marshaling disabled): https://github.com/xamarin/xamarin-macios/blob/2b3ef0e1832cc71adc91eb995cbc7099c123cbe8/tools/common/Application.cs#L101-L112

The idea is to make it possible to enable the generation of these wrapper functions by passing a command-line argument to mtouch/mmp (--optimize=generate-specialized-pinvoke-wrappers for instance).

Numbers for this very simple sample:

void RunTest ()
{
	Stopwatch stopwatch = Stopwatch.StartNew ();
	const int Count = 1000000;
	for (int i = 0; i < Count; i++) {
		var x = RetainCount;
	}
	stopwatch.Stop ();
	Console.WriteLine ($"{Count} iterations in {stopwatch.ElapsedMilliseconds}ms");
}

• Release build (default): 1000000 iterations in 22ms
• Release build with --marshal-objectivec-exceptions=throwmanaged: 1000000 iterations in 235ms
• Release build with --marshal-objectivec-exceptions=throwmanaged and pinvoke wrappers: 1000000 iterations in 22ms

So the effect of exception marshaling is basically nil when generating these invoke wrappers.