arch: introduce initjmp() and Kconfig symbol HAVE_INITJMP

Add the HAVE_INIJMP symbol to be set by architectures that support
initjmp(), a non-standard extension to setjmp()/longjmp() allowing to
initialize a jump buffer with a function pointer and a stack pointer.
This will be useful to later introduce threads. With this new function
it becomes possible to longjmp() to a particular function pointer
(rather than to a point previously reached during program execution as
is the case with setjmp()), and with a custom stack. Both things are
needed to spin off a new thread. Then the usual setjmp()/longjmp() pair
is enough to save and restore a context, i.e., switch thread.

Add the initjmp() prototype to <include/setjmp.h> since it is common to
all architectures.

Add an entry to the API documentation: doc/api/setjmp.rst.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

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