Add single-thread.h header

Add single-thread.h header

This patch move the single-thread syscall optimization defintions from
syscall-cancel.h to new header file single-thread.h and also move the
cancellation definitions from pthreadP.h to syscall-cancel.h.

The idea is just simplify the inclusion of both syscall-cancel.h and
single-thread.h (without the requirement of including all pthreadP.h
defintions).

No semantic changes expected, checked on a build for all major ABIs.

	* nptl/pthreadP.h (CANCEL_ASYNC, CANCEL_RESET, LIBC_CANCEL_ASYNC,
	LIBC_CANCEL_RESET, __libc_enable_asynccancel,
	__libc_disable_asynccancel, __librt_enable_asynccancel,
	__libc_disable_asynccancel, __librt_enable_asynccancel,
	__librt_disable_asynccancel): Move to ...
	* sysdeps/unix/sysv/linux/sysdep-cancel.h: ... here.
	(SINGLE_THREAD_P, RTLD_SINGLE_THREAD_P): Move to ...
	* sysdeps/unix/sysv/linux/single-thread.h: ... here.
	* sysdeps/generic/single-thread.h: New file.
	* sysdeps/unix/sysdep.h: Include single-thread.h.
	* sysdeps/unix/sysv/linux/futex-internal.h: Include sysdep-cancel.h.
	* sysdeps/unix/sysv/linux/lowlevellock-futex.h: Likewise.