Mark internal unistd functions with attribute_hidden [BZ #18822]

Mark internal unistd functions with attribute_hidden to allow direct
access to them within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/unistd.h (__access): Add attribute_hidden.
	(__lseek64): Likewise.
	(__libc_pread64): Likewise.
	(__pipe2): Likewise.
	(__sleep): Likewise.
	(__chdir): Likewise.
	(__fchdir): Likewise.
	(__getcwd): Likewise.
	(__rmdir): Likewise.
	(__execvpe): Likewise.
	(__execve): Likewise.
	(__setsid): Likewise.
	(__getuid): Likewise.
	(__geteuid): Likewise.
	(__getgid): Likewise.
	(__getegid): Likewise.
	(__getgroups): Likewise.
	(__group_member): Likewise.
	(__ttyname_r): Likewise.
	(__isatty): Likewise.
	(__readlink): Likewise.
	(__unlink): Likewise.
	(__gethostname): Likewise.
	(__profil): Likewise.
	(__getdtablesize): Likewise.
	(__brk): Likewise.
	(__ftruncate): Likewise.
	(__ftruncate64): Likewise.