Fix 2 bugs in Debug ROM: (#52)

1. Debug ROM wasn't actually writing 0xffffffff to the last word in
Debug RAM after an exception happened.
2. Fix a race where debug interrupts were cleared before that write
would have happened, so a debugger (gdbserver.cc in this case) might get
the wrong idea about whether an exception happened or not.

Why wasn't this wreaking havoc before?

debug_rom/debug_rom.S

@@ -30,9 +30,6 @@ exception:
 _resume:
         li      s0, 0
 _resume2:
-        # Clear debug interrupt.
-        csrr    s1, CSR_MHARTID
-        sw      s1, CLEARDEBINT(zero)
         fence
 
         # Restore s1.
@@ -40,21 +37,26 @@ _resume2:
         bltz    s1, restore_not_32
 restore_32:
         lw      s1, (DEBUG_RAM + DEBUG_RAM_SIZE - 4)(zero)
-        j       check_halt
+        j       finish_restore
 restore_not_32:
         slli    s1, s1, 1
         bltz    s1, restore_128
 restore_64:
         ld      s1, (DEBUG_RAM + DEBUG_RAM_SIZE - 8)(zero)
-        j       check_halt
+        j       finish_restore
 restore_128:
         nop     #lq      s1, (DEBUG_RAM + DEBUG_RAM_SIZE - 16)(zero)
 
+finish_restore:
         # s0 contains ~0 if we got here through an exception, and 0 otherwise.
         # Store this to the last word in Debug RAM so the debugger can tell if
         # an exception occurred.
         sw      s0, (DEBUG_RAM + DEBUG_RAM_SIZE - 4)(zero)
 
+        # Clear debug interrupt.
+        csrr    s0, CSR_MHARTID
+        sw      s0, CLEARDEBINT(zero)
+
 check_halt:
         csrr    s0, CSR_DCSR
         andi    s0, s0, DCSR_HALT

debug_rom/debug_rom.h

@@ -1,10 +1,10 @@
 static const unsigned char debug_rom_raw[] = {
   0x6f, 0x00, 0x00, 0x06, 0x6f, 0x00, 0xc0, 0x00, 0x13, 0x04, 0xf0, 0xff,
-  0x6f, 0x00, 0x80, 0x00, 0x13, 0x04, 0x00, 0x00, 0xf3, 0x24, 0x40, 0xf1,
-  0x23, 0x20, 0x90, 0x10, 0x0f, 0x00, 0xf0, 0x0f, 0xf3, 0x24, 0x00, 0xf1,
-  0x63, 0xc6, 0x04, 0x00, 0x83, 0x24, 0xc0, 0x43, 0x6f, 0x00, 0xc0, 0x01,
-  0x93, 0x94, 0x14, 0x00, 0x63, 0xc6, 0x04, 0x00, 0x83, 0x34, 0x80, 0x43,
-  0x6f, 0x00, 0xc0, 0x00, 0x13, 0x00, 0x00, 0x00, 0x23, 0x2e, 0x80, 0x42,
+  0x6f, 0x00, 0x80, 0x00, 0x13, 0x04, 0x00, 0x00, 0x0f, 0x00, 0xf0, 0x0f,
+  0xf3, 0x24, 0x00, 0xf1, 0x63, 0xc6, 0x04, 0x00, 0x83, 0x24, 0xc0, 0x43,
+  0x6f, 0x00, 0x80, 0x01, 0x93, 0x94, 0x14, 0x00, 0x63, 0xc6, 0x04, 0x00,
+  0x83, 0x34, 0x80, 0x43, 0x6f, 0x00, 0x80, 0x00, 0x13, 0x00, 0x00, 0x00,
+  0x23, 0x2e, 0x80, 0x42, 0x73, 0x24, 0x40, 0xf1, 0x23, 0x20, 0x80, 0x10,
   0x73, 0x24, 0x00, 0x7b, 0x13, 0x74, 0x84, 0x00, 0x63, 0x04, 0x04, 0x00,
   0x6f, 0x00, 0x80, 0x05, 0x73, 0x24, 0x20, 0x7b, 0x73, 0x00, 0x20, 0x7b,
   0x73, 0x10, 0x24, 0x7b, 0x73, 0x24, 0x00, 0x7b, 0x13, 0x74, 0x04, 0x1c,