mboot64: add exception return logic.

Before I plumb in the exception table support.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: Add handling "unsafe" actions through fixups.

Useful for probing stuff.

Wrap semihosting. Now with semihosting off, you won't crash.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: make md/mw/cmp 64-bit and access-safe.

Should do the rest of these as well at some point..

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: types, printing types

Fix nonsense with u64 definition and add macros
for correctly printing across 32/64-bit.

Can't fix everything, so only fixing it where it's bothering me.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>