Enable ARM M4F FPU lazy stacking

[zephyrbot]

Reported by Maureen Helm:

Initial support for hardware floating point used a simple scheme to always save/restore floating point context. The hardware supports a lazy stacking feature to reduce interrupt latency. This feature is used in both FreeRTOS and mbed RTX.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0298a/index.html

(Imported from Jira ZEP-505)

[ioannisg]

Lazy stacking is now supported in ARM, @MaureenHelm, @carlescufi , so I close this issue.