riscv32: Specify the arch_minimum_kernel as 5.4

Specify the minimum kernel version for RISC-V 32-bit as the 5.4 kernel.
We require this commit: "waitid: Add support for waiting for the current
process group" for the kernel as it adds support for the P_PGID id for
the waitid syscall.  Without this patch we can't replace the wait4
syscall on 64-bit time_t only systems.

Reviewed-by: Maciej W. Rozycki <macro@wdc.com>