arm: Fix CONFIG_RUNTIME_NMI behavior

Zephyr kernel is unable to compile when CONFIG_RUNTIME_NMI is enabled in
defconfig on ARM's architectures.

This patch addresses the following issues:
* In nmi.c _DefaultHandler() is referencing a function
(_ScbSystemReset()) not defined in Zephyr. This has now been replaced
with sys_arch_reboot.
* nmi.h is included in ASM files and due to the usage of "extern" the
compilation ends with an error. Added the directive _ASMLANGUAGE to
prevent the problem.

Jira: ZEP-1319
Change-Id: I7623ca97523cde04e4c6db40dc332d93ca801928
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>