Initial support for dtrace and systemtap

This adds static markers for dtrace, which are also usable
by systemtap. Additionally it adds a tapset for systemtap
that makes it easier to use the static markers.

These are enabled by default.

This initial set of probes is rather limited:

* allocation and free using g_malloc & co
* allocation and free using g_slice
* gquark name tracking (useful for converting quarks to strings in probes)

Notes on naming:

Its traditional with dtrace to use probe names with dashes as
delimiter (slice-alloc). Since dashes are not usable in identifiers
the C code uses double underscores (slice__alloc) which is converted
to dashes in the UI. We follow this for the shared lowlevel probe
names.

Additionally dtrace supports putting a "provider" part in the probe
names which is essentially a namespacing thing. On systemtap this
field is currently ignored (but may be implemented in the future), but
this is not really a problem since in systemtap the probes are
specified by combining the solib file and the marker name, so there
can't really be name conflicts.

For the systemtap tapset highlevel probes we instead use names that
are systemtapish with single dashes as separators.

https://bugzilla.gnome.org/show_bug.cgi?id=606044