riscv: move exception table immediately after RO_DATA

Move EXCEPTION_TABLE immediately after RO_DATA. Make it easy to set the
attribution of the sections which should be read-only at a time.
Add _data to specify the start of data section with write permission.
This patch is prepared for STRICT_KERNEL_RWX support.

Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>