trace: [tcg] Define TCG tracing helper routines

Generates file "trace/generated-helpers.c" with TCG helper definitions to trace
events in guest code at execution time.

The helpers ('helper_trace_${event}_exec_proxy') cast the TCG-compatible native
argument types to their original types (as defined in "trace-events") and call
the tracing routine ('trace_${event}_exec').

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