Make PAL_SEHException smaller (#6267)

Make PAL_SEHException smaller (#6267)

The PAL_SEHException is quite large due to the fact that it contains context
and exception records. This causes a problem when we try to throw it and
system gets out of memory. The C++ runtime can throw exceptions even in that
case, but only if they are smaller than certain threshold. So in our case,
it just aborts the process.

This change separates the context and exception records from the exception,
which ensures that the above mentioned mechanism in the C++ runtime can
kick in and the exception can be thrown even when malloc fails to allocate
it.

I have also modified HandleHardwareException to return BOOL indicating whether
the execution should continue at the possibly modified exception context
instead of using RtlRestoreContext. In that case, we return all the way back
to the signal handler, update the ucontext in there and return from it.