[Backtracing] Add support for building target executables into libexec.

[al45tair]

We're going to add a program, swift-backtrace, that gets built alongside the stdlib and the runtime, and that needs to be installed in libexec/swift alongside the libraries in lib/swift.

It wants to be built with the stdlib/runtime because there's an internal interface between swift-backtrace and the runtime, so the program needs to stay in lock-step with the runtime library.

rdar://105390807

[al45tair]

This is part of a set of stacked PRs:

	PR	Title	Merges Into
👉	#63650	[Backtracing] Add support for building target executables into libexec	main
	al45tair#8	[Backtracing] Add support for looking up paths for auxiliary executables	#63650
	al45tair#9	[Backtracing][Runtime][Darwin] Add code to trigger the external backtracer	al45tair#8
	al45tair#10	[Backtracing] Add _Backtracing library to the build	al45tair#9
	al45tair#11	[Backtracing][Frontend] Add support for implicit import of _Backtracing	al45tair#10
	al45tair#12	[Backtracing] Add swift-backtrace to the build	al45tair#11
	al45tair#13	[Backtracing][Docs] Add an explanation of the workings of the backtracer	al45tair#12
	al45tair#14	[Backtracing] Add control over symbol caching	al45tair#13

(Only the bottom PR will be able to run PR checks at any time. Please don't wait for the checks before review.)

[al45tair]

@swift-ci Please smoke test linux platform

[etcwilde]

Looking, still making sense of what all of the list(APPEND...)s connect to.

[etcwilde]

I think the high-level of where things are going seems sensible to me. I'm not familiar enough with Windows to say on that front. I'm still trying to figure out what an end "happy" state for CMake will look like once I can start using new cmake 3.26 features. I'll keep looking at this tomorrow though.

[etcwilde]

The catalyst stuff makes me sad, but I can't think of any tricks to make them nicer at the moment.

CC @compnerd for comments and thoughts on the Windows side of things.