Remove __libc_waitpid function name.

Continuing the removal of unused __libc_* function names, this patch
removes the __libc_waitpid name.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by the patch; __waitpid, which is exported from
shared libc, changes from weak to strong on some configurations, which
is of no significance).

	* include/sys/wait.h (__libc_waitpid): Remove declaration.
	* posix/waitpid.c (__libc_waitpid): Rename to __waitpid.
	(__waitpid): Don't define as alias.  Use libc_hidden_def not
	libc_hidden_weak.
	(waitpid): Define as alias of __waitpid.
	* sysdeps/unix/bsd/waitpid.c (__libc_waitpid): Rename to
	__waitpid.
	(__waitpid): Don't define as alias.  Use libc_hidden_def not
	libc_hidden_weak.
	(waitpid): Define as alias of __waitpid.
	* sysdeps/unix/sysv/linux/i386/syscalls.list (waitpid): Remove
	__libc_waitpid alias.
	* sysdeps/unix/sysv/linux/m68k/syscalls.list (waitpid): Likewise.
	* sysdeps/unix/sysv/linux/powerpc/syscalls.list (waitpid):
	Likewise.
	* sysdeps/unix/sysv/linux/sh/syscalls.list (waitpid): Likewise.
	* sysdeps/unix/sysv/linux/sparc/syscalls.list (waitpid): Likewise.
	* sysdeps/unix/sysv/linux/tile/waitpid.S (__libc_waitpid): Remove
	alias.
	* sysdeps/unix/sysv/linux/waitpid.c (__libc_waitpid): Rename to
	__waitpid.
	(__waitpid): Don't define as alias.  Use libc_hidden_def not
	libc_hidden_weak.
	(waitpid): Define as alias of __waitpid.