arch: arm: carve only basic stack frame at thread init

The initial carved stack frame only needs to contain the basic
stack frame (state context), because:
- under No/Unshared FP Services mode no FP context is stacked.
- under FP Sharing mode FP context is cleared upon thread
  creation.
This commit removes the unnecessary stacking of the FP context
during thread initialization. This results in reduced stack
usage under the Shared FP Services mode (CONFIG_FP_SHARING=y).
Also, we do not need to initialize the FPSCR, because there is
no FP stack frame carved.

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