Add exception handlers to thread info. And, since we allow threads
running in user mode to install exception handlers too (a convenience
for unit test developers), check for handlers on the user mode stack
thread info too. But, unit test developers will likely also expect the
installation of exception handlers done in kernel mode, before switching
to user mode, to work. So, if there's no handler in the thread info
hanging off the user mode stack, then still check the kernel mode stack
thread info for one.

Use THREAD_SIZE == PAGE_SIZE, when PAGE_SIZE is larger than 16K.
This is for arm64, which uses 64K pages, because the exception
handler arrays are 8K together, making the stack too small with
THREAD_SIZE == 16K.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>