Fix a typo in the RX asse,bler. The Double-precision floating-point exception handling control register name is DECNT not DCENT. * config/rx-parse.y (DECNT): Fixed typo. * testsuite/gas/rx/dpopm.sm (DECNT): Fixed typo. * testsuite/gas/rx/dpushm.sm (DECNT): Fixed typo. * testsuite/gas/rx/macros.inc (DECNT): Fixed typo.