Getting systemtap examples working with --bpf backend

[Frank Ch. Eigler]

wcohen wrote:

> global __gtod_offset = 0 /* FIXME need to set appropriately on startup */

Is there a standard bpf function to get this value?  If so, it's trivial
to call it from a probe-begin and initialize this global.  If not, it's
a less trivial job for it to get a reserved spot in the same global
array where the bpf runtime communicates exit-ness with stapbpf.  Then
stapbpf could initialize this shared global at its startup.  Or a fake
stapbpf-special bpf function could provide this value, and again a
probe begin could save the value.

- FChE