bootm64: flesh out exception handler for aarch64.

This gives more info than just silently dying. E.g.:

RTSM64 # md 1337 beef
00001337:
Unhandled exception from EL2t!
x00: 0000000000001337  x01: 0000000000000000
x02: 0000000000000000  x03: 0000000000000008
x04: 00000000ffffffff  x05: 0000000000000021
x06: 000000008001e800  x07: 000000008001e800
x08: 0000000000000000  x09: ffffffffffffffff
x10: 0000000000000000  x11: 0000000000000002
x12: 0000000000000000  x13: 0000000000000000
x14: 0000000000000000  x15: 0000000000000000
x16: 0000000000000000  x17: 0000000000000000
x18: 0000000000000000  x19: 00000000deaddead
x20: 00000000deaddead  x21: 0000000000000000
x22: 0000000000000000  x23: 0000000000000000
x24: 0000000000000000  x25: 0000000000000000
x26: 0000000000000000  x27: 0000000000000000
x28: 0000000000000000  x29: 000000008fff75e0
x30: 00000000800103b8   sp: 000000008fff75e0
elr: 00000000800103e8 spsr: 00000000600003c9
hpfar: 0000000000000000
far:   0000000000001337
par:   0000000000000000
esr:   0000000096000021

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