feat(cts): Update conformance for POW

Update POW conformance to match the new conformance rules that take into
consideration the latest LOG and EXP conformance and the fix for
subnormals introduced after v1.0.

Add PRECOMPUTED verification mode, that takes the bounds passed by the
Reference Model to be the pre-computed allowable absolute error.

Add unit tests for PRECOMPUTED mode.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I09be3b2dadf7ed9bf7c6c91c64b34c7d4e149868