Fix a number of issues in CMSE examples

[vhscampos]

• Use CLRM to clear registers instead of a sequence of mov rX, #0.
• Save and restore FPCXTS in transitions from Secure to Non-secure and vice-versa.
• Use VSCCLRM to clear floating-point registers if necessary.

Fix for #210

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name: Pull request about: Technical issues, document format problems, bugs in scripts or feature proposal.

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