riscv: add rv32 and rv64 randconfig build targets

Add the ability to do randconfig build targets for both
rv32 and rv64.

Based on a similar patch by Michael Ellerman for PowerPC.

Usage:
  make ARCH=riscv rv32_randconfig
or
  make ARCH=riscv rv64_randconfig

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>