powerpc/64: reuse PPC32 static inline flush_dcache_range()

This patch drops the assembly PPC64 version of flush_dcache_range()
and re-uses the PPC32 static inline version.

With GCC 8.1, the following code is generated:

void flush_test(unsigned long start, unsigned long stop)
{
	flush_dcache_range(start, stop);
}

0000000000000130 <.flush_test>:
 130:	3d 22 00 00 	addis   r9,r2,0
			132: R_PPC64_TOC16_HA	.data+0x8
 134:	81 09 00 00 	lwz     r8,0(r9)
			136: R_PPC64_TOC16_LO	.data+0x8
 138:	3d 22 00 00 	addis   r9,r2,0
			13a: R_PPC64_TOC16_HA	.data+0xc
 13c:	80 e9 00 00 	lwz     r7,0(r9)
			13e: R_PPC64_TOC16_LO	.data+0xc
 140:	7d 48 00 d0 	neg     r10,r8
 144:	7d 43 18 38 	and     r3,r10,r3
 148:	7c 00 04 ac 	hwsync
 14c:	4c 00 01 2c 	isync
 150:	39 28 ff ff 	addi    r9,r8,-1
 154:	7c 89 22 14 	add     r4,r9,r4
 158:	7c 83 20 50 	subf    r4,r3,r4
 15c:	7c 89 3c 37 	srd.    r9,r4,r7
 160:	41 82 00 1c 	beq     17c <.flush_test+0x4c>
 164:	7d 29 03 a6 	mtctr   r9
 168:	60 00 00 00 	nop
 16c:	60 00 00 00 	nop
 170:	7c 00 18 ac 	dcbf    0,r3
 174:	7c 63 42 14 	add     r3,r3,r8
 178:	42 00 ff f8 	bdnz    170 <.flush_test+0x40>
 17c:	7c 00 04 ac 	hwsync
 180:	4c 00 01 2c 	isync
 184:	4e 80 00 20 	blr
 188:	60 00 00 00 	nop
 18c:	60 00 00 00 	nop

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>