Support RISC-V Proxy Kernel

[pavelkryukov]

Follows #918

RISC-V Proxy Kernel is a handler of RISC-V system calls (https://github.com/riscv/riscv-pk).
The ultimate goal is to enable it as one of available MIPT-V kernels, likewise MARS.

The implementation should derive from class Kernel and implement system calls and ELF loading by calling RISC-V PK functions:

https://github.com/riscv/riscv-pk/blob/a1c125dde740f5acd1e20a341cf3426ea44af292/pk/frontend.h#L10

https://github.com/riscv/riscv-pk/blob/a1c125dde740f5acd1e20a341cf3426ea44af292/pk/boot.h#L30

TBD: how to test?