lib: utils/sys: Allow custom HTIF base address for RV32

commit 6dde4358 ("lib: utils/sys: Extend HTIF library to allow custom base address")
forgot to update do_tohost_fromhost() codes for RV32, which still
accesses the HTIF registers using the ELF symbol address directly.

Fixes: 6dde4358

 ("lib: utils/sys: Extend HTIF library to allow custom base address")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Tested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>