You can assume they are the same as most of toolchains would interpret "RV32" target as "RV32I", treating I as a default extension. However you may expected some tools may not work. Using "RV32I" is a clearer message. Although "I" can be considered as a default extension, it has an alternative: the "E" extension ("RV32E") is an I-equivalent for embedded, limited-resources systems. It provides the same instruction set as "I", but only 16, rather than 32 registers.