BZ #15711: Avoid circular dependency for syscall.h

The generated header is compiled with `-ffreestanding' to avoid any
circular dependencies against the installed implementation headers.
Such a dependency would require the implementation header to be
installed before the generated header could be built (See bug 15711).
In current practice the generated header dependencies do not include
any of the implementation headers removed by the use of `-ffreestanding'.

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2013-07-15  Carlos O'Donell  <carlos@redhat.com>

	[BZ #15711]
	* sysdeps/unix/sysv/linux/Makefile ($(objpfx)bits/syscall%h):
	Avoid system header dependency with -ffreestanding.
	($(objpfx)bits/syscall%d): Likewise.