Documentation, HDL code and build tools are available at: github.com/mrisc32.
The following blog posts give some information regarding the architectural features of the MRISC32 ISA:
The MRISC32 Instruction Set Manual (DRAFT) contains more information.
There is currently an MRISC32 port of the GNU toolchain (including GCC, GNU binutils and newlib): mrisc32/mrisc32-gnu-toolchain. It provides: