hurd: Move rtld-strncpy-c.c out of mach/hurd/

There's nothing Mach- or Hurd-specific about it; any port that ends
up with rtld pulling in strncpy will need this.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230319151017.531737-15-bugaevc@gmail.com>

diff --git a/sysdeps/mach/hurd/i386/i686/multiarch/rtld-strncpy-c.c b/sysdeps/i386/i686/multiarch/rtld-strncpy-c.c
similarity index 100%
rename from sysdeps/mach/hurd/i386/i686/multiarch/rtld-strncpy-c.c
rename to sysdeps/i386/i686/multiarch/rtld-strncpy-c.c