riscv: set trap vector earlier

The exception vector of the booting hart is not set before enabling
the mmu and then still points to the value of the previous firmware,
typically _start. That makes it hard to debug setup_vm() when bad
things happen. So fix that by setting the exception vector earlier.

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: yang.zhang <yang.zhang@hexintek.com>
Link: https://lore.kernel.org/r/20240508022445.6131-1-gaoshanliukou@163.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/head.S

@@ -305,6 +305,9 @@ SYM_CODE_START(_start_kernel)
 #else
 	mv a0, a1
 #endif /* CONFIG_BUILTIN_DTB */
+	/* Set trap vector to spin forever to help debug */
+	la a3, .Lsecondary_park
+	csrw CSR_TVEC, a3
 	call setup_vm
 #ifdef CONFIG_MMU
 	la a0, early_pg_dir