target/hexagon: align exceptions for user/sysemu

System mode reports an exception as cs->exception_index = HEX_EVENT_* plus
env->cause_code = HEX_CAUSE_*, but translated code in user mode put the cause
code straight into exception_index, so cpu_loop() was decoding both forms.
gen_exception_decode_fail() and the misaligned-PC check used the raw form
unconditionally, so in system mode the cause code was misread as an event
number.

Use the {event, cause} everywhere and drop the duplicated cases
from cpu_loop(), which fixes HEX_CAUSE_PRIV_USER_NO_SINSN and
HEX_CAUSE_PRIV_USER_NO_GINSN.  The misaligned PC is no longer zeroed
on its way out either, so it reaches the signal frame as si_addr instead
of whatever r31 held.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>