arch: arm: extend use of arch.mode variable in FP sharing mode

Only a single bit of the 4-byte internal (thread.arch) status
variable 'mode' is currently used, when we build with User mode
support (CONFIG_USERSPACE=y). In this commit we extend the usage
of 'mode' variable, adding an additional bit-flag to track the
status of the floating point context in a particular thread,
i.e. to track whether FP context is active or not. The status
bit is meant to be used in context-switch, to restore the FP
register context when required.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>