Hide internal fadvise64/fallocate64 functions [BZ #18822]

Hide internal fadvise64/fallocate64 functions to allow direct access
within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* sysdeps/unix/sysv/linux/posix_fadvise64.c
	(__posix_fadvise64_l64): Add Add libc_hidden_proto and
	libc_hidden_def.
	* sysdeps/unix/sysv/linux/posix_fallocate64.c
	(__posix_fallocate64_l64): Likewise.