Relax INEXACT_ROUND conformance in RESCALE

This version allows the implementation to add `output_zp` using fp32,
while previously conformance could only be met by rounding and casting
to int32_t before output_zp was added.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ib6019db5b402dc328acb5ca0deb1961010364600