riscv,entry: fix misaligned base for excp_vect_table

In RV64, the size of each entry in excp_vect_table is 8 bytes. If the
base of the table is not 8-byte aligned, loading an entry in the table
will raise a misaligned exception. Although such exception will be
handled by opensbi/bbl, this still causes performance degradation.
Signed-off-by: NZihao Yu <yuzihao@ict.ac.cn>
Reviewed-by: NAnup Patel <anup@brainfault.org>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/entry.S

@@ -447,6 +447,7 @@ ENDPROC(__switch_to)
 #endif
 
 	.section ".rodata"
+	.align LGREG
 	/* Exception vector table */
 ENTRY(excp_vect_table)
 	RISCV_PTR do_trap_insn_misaligned