libstdc++: Remove extern "C" from Ryu sources

floating_to_chars.cc includes the Ryu sources into an anonymous
namespace as a convenient way to give all its symbols internal linkage.
But an entity declared extern "C" always has external linkage even
from within an anonymous namespace, so this trick doesn't work in the
presence of extern "C", and it causes the Ryu function generic_to_chars
to be visible from libstdc++.a.

This patch removes the only use of extern "C" from our local copy of
Ryu along with some declarations for never-defined functions that GCC
now warns about.

libstdc++-v3/ChangeLog:

	* src/c++17/ryu/LOCAL_PATCHES: Update.
	* src/c++17/ryu/ryu_generic_128.h: Remove extern "C".
	Remove declarations for never-defined functions.
	* testsuite/20_util/to_chars/4.cc: New test.