[Bug libstdc++/103626] _GLIBCXX_HOSTED should respect -ffreestanding

["arsen at aarsen dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103626

--- Comment #2 from Arsen Arsenović <arsen at aarsen dot me> ---
I started work on marking headers, and I already marked 162 headers (all of
include/{std,bits,backward}, where appropriate) as part of my work on P1642.

This set also lines up with the set of all installed installed headers for
P1642. I already have a list of remaining headers that need to be marked, I'll
be handling these, and maybe reverting the change on <expected> (to make it
freestanding).

The progress can be tracked here: https://github.com/ArsenArsen/gcc/tree/p16412
This branch includes the titular changes, in order to support switching between
freestanding and hosted, a rework of how freestanding headers are handled and
installed (though, I would like to exclude even more targets and files, so that
stuff doesn't get stamped needlessly, probably by constraining allstamped,
unsure so far), <bits/requires_hosted.h>, and the aforementioned marking.

Next on my agenda is checking whether this version still produces equivalent
results in hosting to the unpatched version of GCC, and updating the testsuite
after that, then I can resume marking headers if this approach is deemed
acceptable (which I'd like to get a greenlight on before editing 400-some
headers by hand)