hurd: Do not make sigprocmask available in ld.so

After commit f7649d57 ("dlopen: Do not
block signals"), the dynamic linker no longer uses sigprocmask, which
means that it does not have to be made available explicitly on hurd.

This reverts commit 892badc9
("hurd: Make __sigprocmask GLIBC_PRIVATE") and commit
d5ed9ba2 ("hurd: Fix ld.so link"),
but keeps the comment changes from the second commit.