arch: arm: add internal structure to ESF

The ARM Cortex-M Exception Stack Frame (ESF) may consist of
several stack frame contexts (basic state context, additional
state context, FP context, etc.). To reflect these structural
properties, this commit re-factors the ESF, splitting out the
basic stack frame, holding the state context, into its own
struct container. The commit does not introduce behavioral
changes.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>