armv8: shrink exception table code

armv8: shrink exception table code

In the moment our exception entry code needs 34 instructions, so we
can't use put it directly into the table entry, which offers "only"
32 instructions there. Right now we just put an unconditional branch
there, then use a macro to place the 34 instructions *per entry* after
that. That effectivly doubles the size of our exception table, which
is quite a waste, given that we use it mostly for debugging purposes.

Since the register saving part is actually identical, let's just convert
that macro into a function, and "bl" into it directly from the exception
slot, of course after having saved at least the original LR.
This saves us about 950 bytes of code, which is quite a relief for some
tight SPLs, in particular the 64-bit Allwinner ones.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>