[3.14] gh-142779: Initialize reserved field for proper padding (GH-142780) (#144159)

gh-142779: Initialize reserved field for proper padding (GH-142780)

The jitdump specification specifies a reserved field for padding.

Initialize it so no garbage data is embedded in the jitdump files.
(cherry picked from commit 77bf4ba732a8736910bd6b7ec5a58b8ee833c95f)

Co-authored-by: stratakis <cstratak@redhat.com>