libstdc++: Enable building libstdc++.{a,so} when !HOSTED

This enables us to provide symbols for placeholders and numeric limits,
and allows users to mess about with linker flags less.

libstdc++-v3/ChangeLog:

	* Makefile.am [!_GLIBCXX_HOSTED]: Enable src/ subdirectory.
	* Makefile.in: Regenerate.
	* src/Makefile.am [!_GLIBCXX_HOSTED]: Omit compatibility files.
	There's no history to be compatible with.
	* src/c++11/Makefile.am [!_GLIBCXX_HOSTED]: Omit hosted-only
	source files from the build.
	* src/c++17/Makefile.am [!_GLIBCXX_HOSTED]: Likewise.
	* src/c++20/Makefile.am [!_GLIBCXX_HOSTED]: Likewise.
	* src/c++98/Makefile.am [!_GLIBCXX_HOSTED]: Likewise.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>