Error when building Zephyr example

Hi,

I'm following the steps stated in README.md but can't get zephyr/samples/basic/blinky to build. I get really strange error about unrecognized opcodes.

```
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:124: Error: unrecognized opcode `csrrc a4,mstatus,a4'
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:141: Error: unrecognized opcode `csrrs a4,mstatus,a4'
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:124: Error: unrecognized opcode `csrrc s0,mstatus,s0'
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:141: Error: unrecognized opcode `csrrs s0,mstatus,s0'
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:124: Error: unrecognized opcode `csrrc a5,mstatus,a5'
/home/aleksa/RISCV/swervolf/zephyr/include/arch/riscv/arch.h:141: Error: unrecognized opcode `csrrs a5,mstatus,a5'
ninja: build stopped: subcommand failed.
FATAL ERROR: command exited with status 1: /usr/bin/cmake --build /home/aleksa/RISCV/swervolf/zephyr/samples/basic/blinky/build

```

this is the last couple of lines of output.

The command I'm using is
`west build -b swervolf_nexys`

Any help is appreciated.