Hide internal __mremap function [BZ #18822]

Hide internal __mremap function [BZ #18822]

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.