fix(asm): scale the FLDPQ and FSTPQ pair offsets by sixteen

The pair encoder derived the imm7 divisor from the width suffix alone, so
the 128-bit FP pairs divided their offsets by eight and encoded twice the
distance.  The Q spellings scale by sixteen like every other 128-bit
access; the differential kernel carries them now.

Assisted-by: GLM 5.3 Flash

testdata/verify/atomics_arm64.s

@@ -69,4 +69,6 @@ TEXT ·pairs(SB), NOSPLIT, $0-0
 	FLDPD (R8), (F1, F2)
 	FLDPD 8(R8), (F3, F4)
 	FSTPD (F3, F4),-8(R9)
+	FLDPQ 16(R8), (F1, F2)
+	FSTPQ (F1, F2), 48(R9)
 	RET