Fix exception handling in masm backend

Fix exception handling in masm backend

The return value of setjmp is an int, which we interpret as a boolean later. We
cannot put it directly into a VM::Value, because it isn't one. Instead we have
to store it properly with value and tag.

This worked by "accident" on x86-64 where the return value is in eax and not
clobbered until we do the type test. But on ia32 we do rely on a proper value
representation.

Change-Id: Id54072c31baf6ad4607626dad939c715d2092d3d
Reviewed-by: Lars Knoll <lars.knoll@digia.com>