sparc: Fix arch_fork definition

This patch fixes 3dc21497 for sparc.  Different than other architectures
SPARC kernel Kconfig does not define CONFIG_CLONE_BACKWARDS, however it
has the same ABI as if it did, implemented by sparc-specific code
(sparc_do_fork).

It also has a unique return value convention for clone:

   Parent -->  %o0 == child's  pid, %o1 == 0
   Child  -->  %o0 == parent's pid, %o1 == 1

Which required a special macro to correct issue the syscall
(INLINE_CLONE_SYSCALL).

Checked on sparc64-linux-gnu and sparcv9-linux-gnu.

	* sysdeps/unix/sysv/linux/arch-fork.h [__ASSUME_CLONE_BACKWARDS]
	(arch_fork): Issue INLINE_CLONE_SYSCALL if defined.
	* sysdeps/unix/sysv/linux/sparc/kernel-features.h
	(__ASSUME_CLONE_BACKWARDS): Define.