trace: Provide a per-event status define for conditional compilation

trace: Provide a per-event status define for conditional compilation

Adds a 'TRACE_${NAME}_ENABLED' preprocessor define for each tracing event in
"trace.h".

This lets the user conditionally compile code with a relatively high execution
cost that is only necessary when producing the tracing information for an event
that is enabled.

Note that events using this define will probably have the "disable" property by
default, in order to avoid such costs on regular builds.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>