powerpc/64: Implement clear_bit_unlock_is_negative_byte()

Commit b91e1302 ("mm: optimize PageWaiters bit use for
unlock_page()") added a special bitop function to speed up
unlock_page(). Implement this for 64-bit powerpc.

This improves the unlock_page() core code from this:

	li	9,1
	lwsync
1:	ldarx	10,0,3,0
	andc	10,10,9
	stdcx.	10,0,3
	bne-	1b
	ori	2,2,0
	ld	9,0(3)
	andi.	10,9,0x80
	beqlr
	li	4,0
	b	wake_up_page_bit

To this:

	li	10,1
	lwsync
1:	ldarx	9,0,3,0
	andc	9,9,10
	stdcx.	9,0,3
	bne-	1b
	andi.	10,9,0x80
	beqlr
	li	4,0
	b	wake_up_page_bit

In a test of elapsed time for dd writing into 16GB of already-dirty
pagecache on a POWER8 with 4K pages, which has one unlock_page per 4kB
this patch reduced overhead by 1.1%:

    N           Min           Max        Median           Avg        Stddev
x  19         2.578         2.619         2.594         2.595         0.011
+  19         2.552         2.592         2.564         2.565         0.008
Difference at 95.0% confidence
	-0.030  +/- 0.006
	-1.142% +/- 0.243%

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Made 64-bit only until I can test it properly on 32-bit]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>