[PATCH] powerpc: implement missing jiffies64_to_cputime64()

asm-powerpc/cputime.h doesn't declare jiffies64_to_cputime64() or
cputime64_sub(), and due to CONFIG_VIRT_CPU_ACCOUNTING it's not picking
up the definition from asm-generic like x86-64 & friends do.

Cc: Dave Jones <davej@redhat.com>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>