Speed up stack trace generation for the JIT

It turns out that in QML it is not unusual that during early binding
evaluations due to the undefined order, the evaluation tries to look up
properties in objects that aren't initialized yet and thus exceptions are
thrown. Eeach thrown exception saves a stack trace, which is expensive to
generate when using the JIT, as it does full stack unwinding.

This patch implements a more light-weight approach by storing the instruction
pointer in the context before leaving JIT generated code.

Change-Id: I95e1cfd01179247dfc2c1df949828f474a23161b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>