libstdc++: Rework how freestanding install works [PR106953]

libstdc++: Rework how freestanding install works [PR106953]

In light of there being far more freestanding headers now, ad-hoc
maintenance of a subset of the install implementation has become
unsustainable. Instead, we gate off a part of the normal install routine
so that it works without HOSTED enabled, as well as subdivide lists of
headers into freestanding and hosted components, according to the HOSTED
flag.

libstdc++-v3/ChangeLog:

	PR libstdc++/106953
	* include/Makefile.am [!_GLIBCXX_HOSTED]: Remove
	install-freestanding-headers, unifying it with the usual
	install-headers
	* include/Makefile.in: Regenerate.