Use osv::run in run_elf()

In run_elf(), our JNI function for running an executable (used, for example
in the shell's "run" command), use osv::run instead of reimplementing it.
osv::run has recently acquired more steps, namely copying the arguments
to be consecutive in memory, and in the future might get even more complex,
e.g., treat executables differently from shared objects. So let's use
osv::run when we can.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>