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>

arch/riscv/kernel/vmlinux.lds.S

@@ -58,6 +58,10 @@ SECTIONS
 		*(.srodata*)
 	}
 
+	EXCEPTION_TABLE(0x10)
+
+	_data = .;
+
 	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)
 	.sdata : {
 		__global_pointer$ = . + 0x800;
@@ -68,8 +72,6 @@ SECTIONS
 
 	BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
 
-	EXCEPTION_TABLE(0x10)
-
 	.rel.dyn : {
 		*(.rel.dyn*)
 	}