[runtimes] Revert the libc++ __config_site change

This is a massive revert of the following commits (from most revent to oldest):

	2b9b7b57.
	529ac331
	28270234
	69c20872
	b5aa6744
	5d796645

After checking-in the __config_site change, a lot of things started breaking
due to widespread reliance on various aspects of libc++'s build, notably the
fact that we can include the headers from the source tree, but also reliance
on various "internal" CMake variables used by the runtimes build and compiler-rt.

These were unintended consequences of the change, and after two days, we
still haven't restored all the bots to being green. Instead, now that I
understand what specific areas this will blow up in, I should be able to
chop up the patch into smaller ones that are easier to digest.

See https://reviews.llvm.org/D89041 for more details on this adventure.