riscv: Fix 32b kernel build with CONFIG_DEBUG_VIRTUAL=y

Declare kernel_virt_addr for 32b kernel since it is used in
__phys_addr_symbol defined when CONFIG_DEBUG_VIRTUAL is set.

Fixes: 2bfc6cd8

 ("riscv: Move kernel mapping outside of linear mapping")
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>