Arm backend: Refactor compile spec handling (#14111)

Instead of factories to create list[CompileSpec], use a dataclass that
contains all specs directly. This cleans up the API for creating
CompileSpecs and aligns it with how the partitioner and quantizer is
designed. Keep compile spec in easily handled ArmCompileSpec format as
long as possible, only go to list[CompileSpec] format when neccessary
for the backend API. ArmCompileSpec is subclassed for each backend to
minimize risk of using incorrect compile spec for a specific target.

Signed-off-by: Erik Lundell <erik.lundell@arm.com>

Author: Erik-Lundell