freertos stacking: fixed alignment and added support for cm4

This fixes:
- freertos: detection of padded stack frames by evaluating xPSR

This adds:
- freertos: support for cm4
- freertos: support for cm4 with fpu enabled

Change-Id: I443c46cc9daf993010b9a9aff918c7adff99f02f
Signed-off-by: Vincent Kessler <lilvinz@nscit.com>