arm64: AArch32 user space PC alignment exception

ARMv7 does not have a PC alignment exception. ARMv8 AArch32
user space however can produce a PC alignment exception. Add
handler so that we do not dump an unexpected stack trace in
the logs.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>