ARM: FPU: implement ASSERT-based testing of multi-usage in Unshared FP Services mode

[ioannisg]

We may want to enhance the implementation of Unshared FP Services' mode, by providing a testing mechanism that detects corruptions of FPU as a result of accidental sharing between multiple thread contexts.

The mechanism should trigger ASSERTS if the FPU is used by multiple threads under the Unshared Mode (CONFIG_FLOAT=y, CONFIG_FP_SHARING=n).