kernel: fatal: Fix NO_OPTIMIZATIONS build

When logging is on and optimization and multithreading is off then
build fails to link because unoptimized compiler/linker seems to not
look beyond the function and it fails trying to link k_thread_name_get.
Reworking the code to make it known to the compiler without optimization
that k_thread_name_get is not needed and not logging current thread
name in that case.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>