hppa: fix sysdep.h header setup

hppa: fix sysdep.h header setup

The semi-recent SYSCALL_CANCEL inclusion broke hppa due to the sysdep.h
headers not including the unix/sysdep.h headers.  Rework the includes so
we match the other ports:
* hppa/sysdep.h:
	- Do not include sys/syscall.h as the unix sysdep.h headers do it.
	- Do not include config.h as libc-symbols.h does it, and it has no
	#ifdef multiple-include protection, and it breaks when some files
	do things like #undef __OPTIMIZE__.
* sysdeps/unix/sysv/linux/hppa/sysdep-cancel.h:
	- Drop the generic/sysdep.h as the unix sysdep.h headers include it.
* sysdeps/unix/sysv/linux/hppa/sysdep.h:
	- Change to the unix & core hppa sysdep header stacks.
	- Undef a few defines that the core headers already set up for us.