Add Alias to MCXT_NUM_SIMD_SLOTS
Since MCXT_NUM_SIMD_SLOTS is often used to obtain the number of supported SIMD vector registers, we should add an alias, namely DR_NUM_SIMD_REGISTERS, for better code clarity.
Since MCXT_NUM_SIMD_SLOTS is often used to obtain the number of supported SIMD vector registers, we should add an alias, namely DR_NUM_SIMD_REGISTERS, for better code clarity.