Arm backend: Refactor compile spec handling

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.

TODO: go back to ArmCompileSpec format in our backend classes.

Signed-off-by: Erik Lundell <[email protected]>
Change-Id: I90f6e7bc8cbfd5d248da728f401c8ae1fc116dab

Author: Erik-Lundell