Hide internal __mremap function to allow direct access within libc.so and
libc.a without using GOT nor PLT.
__GI___mremap is defined when sysdeps/unix/syscalls.list is used to
generate mremap. Otherwise libc_hidden_def is needed explicitly.
[BZ #18822]
* include/sys/mman.h (__mremap): Add libc_hidden_proto.
* sysdeps/unix/sysv/linux/m68k/mremap.S (__mremap): Add
libc_hidden_def.