CMSE secure state may leak from stack to floating-point registers

When using Arm Cortex-M Security Extensions (CMSE), Secure stack

contents can be leaked to Non-secure state via floating-point registers

when a Secure to Non-secure function call is made that returns a

floating-point value and when this is the first use of floating-point

since entering Secure state. This allows an attacker to read a limited

quantity of Secure stack contents with an impact on confidentiality.

This issue is specific to code generated using LLVM-based compilers.