Revert "RT4526: Call TerminateProcess, not ExitProcess"

This reverts commit 9c1a9ccf

.

TerminateProcess is asynchronous, so the code as written in the above
commit is not correct. It is also probably not needed in the speed
case. Reverting in order to figure out the correct solution.

Reviewed-by: Rich Salz <rsalz@openssl.org>