display assembly instructions, to help with first.tex exercise

.gdbinit.tmpl-riscv

@@ -2,3 +2,4 @@ set confirm off
 set architecture riscv
 target remote 127.0.0.1:1234
 symbol-file kernel/kernel
+set disassemble-next-line auto

kernel/trampoline.S

@@ -12,6 +12,7 @@
 	.section trampoline
 .globl trampout
 trampout:
+        # trampout(trapframe, pagetable)
         # switch from kernel to user.
         # usertrapret() calls here.
 	# a0: p->tf in user page table

kernel/trap.c

@@ -117,7 +117,7 @@ usertrapret(void)
   // set S Exception Program Counter to the saved user pc.
   w_sepc(p->tf->epc);
 
-  // tell trampline.S the user page table to switch to.
+  // tell trampoline.S the user page table to switch to.
   uint64 satp = MAKE_SATP(p->pagetable);
 
   // jump to trampoline.S at the top of memory, which