aarch64: rationalize exception entry/exit code

Each vector slot has room for 32 instructions. The exception context
saving needs 15 instructions already. Rather than duplicating those
instructions in each out-of-line exception routines, let's store
them directly in the vector table. That vector space is otherwise
wasted anyway. Move the z_arm64_enter_exc macro into vector_table.S
as this is the only place where it should be used.

To further reduce code size, let's make z_arm64_exit_exc into a
function of its own to avoid code duplication again. It is put in
vector_table.S as this is the most logical location to go with its
z_arm64_enter_exc counterpart.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>