Rename bits/m68k-vdso.h to m68k-vdso.h (bug 14912).

It was noted in
<https://sourceware.org/ml/libc-alpha/2012-09/msg00305.html> that the
bits/*.h naming scheme should only be used for installed headers.
This patch renames bits/m68k-vdso.h to plain m68k-vdso.h to follow
that convention.

	[BZ #14912]
	* sysdeps/unix/sysv/linux/m68k/bits/m68k-vdso.h: Move to ...
	* sysdeps/unix/sysv/linux/m68k/m68k-vdso.h: ...here.
	* sysdeps/unix/sysv/linux/m68k/coldfire/bits/atomic.h: Include
	<m68k-vdso.h> instead of <bits/m68k-vdso.h>.
	* sysdeps/unix/sysv/linux/m68k/init-first.c: Likewise.
	* sysdeps/unix/sysv/linux/m68k/m68k-helpers.S: Likewise.
	* sysdeps/unix/sysv/linux/m68k/m68k-vdso.c: Likewise.