Issue Description:

The stdlib CMake config defines a local variable `swiftlib_link_flags_all`, then adds some link flags for Apple platforms and ELF platforms. However, a recent single-line change resets that variable and drops those flags, added with the Catalyst changes in January.

I notified the author of that pull (never got a response to multiple pings on github) and got a pull merged that reverted that single line reset in April, but that was quickly reverted because of "an internal CI failure" at Apple, presumably why the Catalyst pull dropped these flags in the first place.

Someone needs to go through and figure out which of these linker flags are still needed and correct this now dead code. Not going to be me, as I can't even get these Apple engineers to respond to me.