Add shim header for bits/syscall.h.

Add shim header for bits/syscall.h.

On Linux-based configurations, bits/syscall.h is a generated file.
To avoid build-ordering problems, the Linux sys/syscall.h only includes
bits/syscall.h if _LIBC is not defined.  After the _ISOMAC-testsuite
changes, this means any test case that includes sys/syscall.h tries to
pull in bits/syscall.h.  This would be fine, because it'll definitely
have been generated by the time we start compiling tests, except that
the generated <builddir>/misc/bits/syscall.h is not visible in the
include path, because nothing needed it till now.  So we either get
the bits/syscall.h from the host system, or the build fails.

The fix is simple: add a shim header for bits/syscall.h.  I put it in
sysdeps/unix/sysv/linux/include instead of the top-level include/
because bits/syscall.h doesn't exist at all on other configurations as
far as I can tell.

This is known to affect nptl/tst-cond2[45].  Thanks to John David
Anglin for noticing the problem.

	[BZ #21514]
	* sysdeps/unix/sysv/linux/include/bits/syscall.h: New shim header
	pointing to the generated file in <builddir>/misc/bits/syscall.h.