Revert "RT4526: Call TerminateProcess, not ExitProcess"

This reverts commit 75f90688.

TerminateProcess is asynchronous, so the code as written in the above
commit is not correct (and doesn't even compile at the moment). 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>