Hexagon (target/hexagon) Clean up handling of modifier registers

Hexagon (target/hexagon) Clean up handling of modifier registers

Currently, the register number (MuN) for modifier registers is the
modifier register number rather than the index into hex_gpr.  This
patch changes MuN to the hex_gpr index, which is consistent with
the handling of control registers.

Note that HELPER(fcircadd) needs the CS register corresponding to the
modifier register specified in the instruction.  We create a TCGv
variable "CS" to hold the value to pass to the helper.

Reviewed-by: Brian Cain <bcain@quicinc.com>
Signed-off-by: Taylor Simpson <ltaylorsimpson@gmail.com>
Message-Id: <20231210220712.491494-2-ltaylorsimpson@gmail.com>
Signed-off-by: Brian Cain <bcain@quicinc.com>