lib: cpp: Rename CONFIG_EXCEPTIONS to CONFIG_CPP_EXCEPTIONS

`CONFIG_EXCEPTIONS` is ambiguous in that the word "exception" may be
used to refer to many different types of exceptions, from the hardware
generated faults and interrupts (aka. exceptions) to the C++
exceptions.

This commit renames the Kconfig `CONFIG_EXCEPTIONS` symbol to
`CONFIG_CPP_EXCEPTIONS` in order to clarify that the symbol refers to
the C++ exceptions and eliminate any ambiguity, and deprecates the old
`CONFIG_EXCEPTIONS` symbol.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>