armv8: Reduce exception handling code

armv8: Reduce exception handling code

The arm64 exception handling code is quite big, mostly due to
architectural alignment requirements. Each exception entry spans 32
instructions, which sounds generous, but is too small to fit all of the
save/branch/restore code in there. So at the moment we use only four
instructions, branching into shared save and restore routines.
To not leave the space for those remaining 28 instructions wasted, let's
split the save and restore routines and stuff them into the gaps.
This saves about 250 bytes of code, which is helpful for those tight
SPLs.

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