ARC: fault: Fix uninitialized memory access

Found via static analysis. In fault path when checking for stack
overflows, if CONFIG_MULTITHREADING is not set, `guard_end` is left
uninitialized and is subsequently used in a comparison.

The solution is to simply return `false` in this configuration as stack
guards are not configured in the first place.

Signed-off-by: François Baldassari <francois@memfault.com>