hurd: Move libc_hidden_def's around

Each libc_hidden_def should be placed immediately next to its function,
not in some random unrelated place.

No functional change.

Fixes: 653d74f1
"hurd: Global signal disposition"

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230429131354.2507443-2-bugaevc@gmail.com>