arch: arm: major cleanup and refactoring for fault function

This commit refactors and cleans up __fault, so the function
- reduces to supplying MSP, PSP, and EXC_RETURN to the C
  function for fault handling
- simplifies itself, removing conditional
  implementation, i.e. based on ARM Secure firmware,

The reason for that is simple: it is much better to write the
fault handling in C instead of assembly, so we really do only
what is strictly required, in assembly.

Therefore, the commit refactors the z_arm_fault() function
as well, organizing better the different functional blocks,
that is:
- unlocking interrupts
- retriving ESF
- asserting for HW errors
- printing additional error logs

The refactoring unifies the way the ESF is retrieved for the
different Cortex-M variants and security execution states.

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