Kernel: Add exception_code to RegisterDump.

Added the exception_code field to RegisterDump, removing the need
for RegisterDumpWithExceptionCode. To accomplish this, I had to
push a dummy exception code during some interrupt entries to properly
pad out the RegisterDump. Note that we also needed to change some code
in sys$sigreturn to deal with the new RegisterDump layout.

Kernel/Arch/i386/PIT.cpp

@@ -12,6 +12,7 @@ extern "C" void timer_interrupt_handler(RegisterDump&);
 asm(
     ".globl timer_interrupt_entry \n"
     "timer_interrupt_entry: \n"
+    "    pushl $0x0\n"
     "    pusha\n"
     "    pushw %ds\n"
     "    pushw %es\n"
@@ -34,6 +35,7 @@ asm(
     "    popw %es\n"
     "    popw %ds\n"
     "    popa\n"
+    "    add $0x4, %esp\n"
     "    iret\n");
 
 static u32 s_ticks_this_second;