Fix: cache the result of getpid() internally

Fix: cache the result of getpid() internally

On Linux we called getpid() directly on each tracepoint and relied on
the glibc pid cache. However, in glibc 2.25, released on 2017-02-05, the
pid cache was removed which results in a getpid syscall on each event
when the vpid context is enabled.

Remove the Linux specific case and use our internal cache all the time.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
(cherry picked from commit 6d9b54d845b08d54d3f483f1130b499516588db3)

Change-Id: Ide9a7f3c99061e3b3d9f68cec86fc117dc684be7