riscv: Update alignment for some sections in linker scripts

Some sections in the linker scripts are aligned to 4 bytes, which
may cause misaligned exception on some platforms, e.g.: clearing
the bss section on 64-bit hardware if __bss_start does not start
from a naturally 8 bytes aligned address.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>