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kernel: more (arm) compiler friendly mul64fract()

the arm compiler can lift long->vlong casts on multiplcation
and convert 64x64->64 multiplication into a 32x32->64 one
with optional 64 bit accumulate.

--- a/sys/src/9/port/mul64fract.c

+++ b/sys/src/9/port/mul64fract.c

@@ -21,19 +21,15 @@

 void

 mul64fract(uvlong *r, uvlong a, uvlong b)

 {

-	uvlong bh, bl;

-	uvlong ah, al;

-	uvlong res;

+	ulong bh, bl, ah, al;

-	bl = b & 0xffffffffULL;

+	bl = b;

 	bh = b >> 32;

-	al = a & 0xffffffffULL;

+	al = a;

 	ah = a >> 32;

-	res = (al*bl)>>32;

-	res += (al*bh);

-	res += (ah*bl);

-	res += (ah*bh)<<32;

-

-	*r = res;

+	*r = (((uvlong)al*(uvlong)bl)>>32)

+	   + ((uvlong)al*(uvlong)bh)

+	   + ((uvlong)ah*(uvlong)bl)

+	   + (((uvlong)ah*(uvlong)bh)<<32);

 }