libstdc++: Avoid useless dependency on read_symlink from tzdb

chrono::tzdb::current_zone uses filesystem::read_symlink, which creates
a dependency on the fs_ops.o object in libstdc++.a, which then creates
dependencies on several OS functions if --gc-sections isn't used. For
more details see PR libstdc++/104167 comment 8 and comment 11.

In the cases where that causes linker failures, we probably don't have
readlink anyway, so the filesystem::read_symlink call will always fail.
Repeat the preprocessor conditions for filesystem::read_symlink in the
body of chrono::tzdb::current_zone so that we don't create a
dependency on fs_ops.o for a function that will always fail.

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (tzdb::current_zone): Check configure macros
	for POSIX readlink before using filesystem::read_symlink.

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