sysinit: pointee type should be u8 when using ptr::write_bytes
Using *const (), no bytes are written to the position, which might
result in uninitialized memory access.
Fixes: ebd3d1224c01 ("change(x86): optimize bootstrap code, remove kinit.cpp")
Fixes: 191877a3acd0 ("feat(hal): impl basic single hart bootstrap for riscv64")
Signed-off-by: greatbridf <greatbridf@icloud.com>