build: use zero overhead systemtap probes
Profiling suggested that on Linux sometimes over 10% of CPU time was
being spent inside the systemtap probe entry points in the binding
layer, even when the process was not actively being traced with the
`stap` tool.
That's why this commit makes it possible to use the *_ENABLED() macros
and bail out early when we're not being traced, reducing the overhead
of unused probes to (almost) zero.
Said macros were already being generated by `dtrace -h` but were not
usable because they rely on external definitions. To remedy that, we
now generate the accompanying object files with `dtrace -G`.
This commit includes a change to libuv that has been landed upstream in
commit joyent/libuv@3c172ea.