target/hexagon: rename HEX_EXCP_*=>HEX_CAUSE_*

target/hexagon: rename HEX_EXCP_*=>HEX_CAUSE_*

The values previously used for "HEX_EXCP_*" were the cause code
definitions and not the event numbers.  So in this commit, we update
the names to reflect the cause codes. In HEX_EVENT_TRAP0's case, we add
a new "HEX_EVENT_*" with the correct event number.

Reviewed-by: Taylor Simpson <ltaylorsimpson@gmail.com>
Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>