Older versions of newlib do not provide truncate so add a configure
check for it, and provide a fallback definition.

There were also some missing exports in the linker script, which went
unnoticed because there are no tests for some functions. A new link-only
test checks that every filesystem operation function is defined by the
library.

	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check for truncate.
	* config.h.in: Regenerate.
	* config/abi/pre/gnu.ver: Order patterns for filesystem operations
	alphabetically and add missing entries for copy_symlink,
	hard_link_count, rename, and resize_file.
	* configure: Regenerate.
	* src/c++17/fs_ops.cc (resize_file): Remove #if so posix::truncate is
	used unconditionally.
	* src/filesystem/ops-common.h (__gnu_posix::truncate)
	[!_GLIBCXX_HAVE_TRUNCATE]: Provide fallback definition that only
	supports truncating to zero length.
	* testsuite/27_io/filesystem/operations/all.cc: New test.
	* testsuite/27_io/filesystem/operations/resize_file.cc: New test.

From-SVN: r267647