Arm backend: Legalize unsigned RESCALE from wide inputs (#21393)

TOSA RESCALE only allows output_unsigned when the input is int8 or
int16. Some preserved uint8 IO paths can produce a final wide
accumulator RESCALE with output_unsigned set, which newer
model-converter builds reject.

Canonicalize that case during TOSA serialization by emitting a signed
RESCALE to the target integer dtype first, followed by a legal
signed-input to unsigned-output boundary RESCALE.


cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani

Signed-off-by: Baris Demir <baris.demir@arm.com>