Tracing: TRACING_NONE Compilation Fix

Fixed several compilation errors that resulted from selecting
TRACING without specifying a tracing system (Tracerecorder,
CTF, Systemview). In this case (TRACING_NONE), some default trace hooks
(in tracing.h) were incorrectly named resulting in compilation errors.
The legacy sys_trace_isr_enter, sys_trace_isr_exit, and sys_trace_idle
also caused problems since these were only given as defines, resulting
in undefined reference errors since they are required by the assembly
files calling these. To solve this issue I've added a stub file
"tracing_none.c" (only compiled if TRACING_NONE) and declared the
functions in tracing.h if no tracing system is selected.

Signed-off-by: Torbjörn Leksell <torbjorn.leksell@percepio.com>