Add loadavg tapset

Add loadavg tapset

Provide a tapset to obtain the load average.

Sample usage via a timer probe:

	probe timer.s(1) {
		printf("%s: Load average: %s",
			ctime(gettimeofday_s()),
			sprint_loadavg())
	}

$ sudo stap ./show_loadavg.stp
Tue Nov 26 16:54:04 2013: Load average: 0.12 0.33 0.30
Tue Nov 26 16:54:05 2013: Load average: 0.11 0.33 0.30
Tue Nov 26 16:54:06 2013: Load average: 0.11 0.33 0.30
Tue Nov 26 16:54:07 2013: Load average: 0.11 0.33 0.30
Tue Nov 26 16:54:08 2013: Load average: 0.11 0.33 0.30
Tue Nov 26 16:54:09 2013: Load average: 0.11 0.33 0.30
Tue Nov 26 16:54:10 2013: Load average: 0.18 0.34 0.30
Tue Nov 26 16:54:11 2013: Load average: 0.18 0.34 0.30

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>