tracing: add global-clock option to provide cross CPU clock to traces

Impact: feature to allow better serialized clock

This patch adds an option called "global-clock" that will allow
the tracer to switch to a slower but more accurate (across CPUs)
clock.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

diff --cc kernel/trace/trace.c
Simple merge

diff --cc kernel/trace/trace.h
Simple merge