libstdc++: Fix libstdc++exp.a so it really does contain Filesystem TS symbols

In r14-3812-gb96b554592c5cb I claimed that libstdc++exp.a now contains
all the symbols from libstdc++fs.a as well as libstdc++_libbacktrace.a,
but that wasn't true. Only the symbols from the latter were added to
libstdc++exp.a, the Filesystem TS ones weren't. This seems to be because
libtool won't combine static libs that are going to be installed
separately. Because libstdc++fs.a is still installed, libtool decides it
shouldn't be included in libstdc++exp.a.

The solution is similar to what we already do for libsupc++.a: build two
static libs, libstdc++fs.a and libstdc++fsconvenience.a, where the
former is installed and the latter isn't. If we then tell libtool to
include the latter in libstdc++exp.a it will do as it's told.

libstdc++-v3/ChangeLog:

	* src/experimental/Makefile.am: Use libstdc++fsconvenience.a
	instead of libstdc++fs.a.
	* src/experimental/Makefile.in: Regenerate.
	* src/filesystem/Makefile.am: Build libstdc++fsconvenience.a as
	well.
	* src/filesystem/Makefile.in: Regenerate.