remote: workaround to debug x86_64 architecture for the linux kernel

[daeseokyoun]

This didn't seem to be fixed since 2012 Mar.
Attach gdbserver on qemu from client and set a breakpoint in
start_kernel function but it will be failed with below error:
(gdb) c
Remote 'g' packet reply is too long: 00004082fffffff
...

Refer https://wiki.osdev.org/QEMU_and_GDB_in_long_mode OS-Dev wiki
to fix issue with workaround patch.

My Scenario for kernel debugging

• Build binutild-gdb after apply the patch for fixing issue

$ ./configure
$ make -j8 && sudo make install

• the gdb binary will be installed in /usr/local/bin/gdb

• Run qemu for x86_64 architecture

$ qemu-system-x86_64 -s -S -kernel arch/x86/boot/bzImage -boot c -m 2049M \
   -hda ../buildroot/output/images/rootfs.ext2 \
   -append "root=/dev/sda rw console=ttyS0,115200 acpi=off nokaslr" \
   -serial stdio -display none

• Run gdb with ./vmlinux

$ cd /path/to/linux/kernel/top
$ gdb ./vmlinux
(gdb) target remote :1234
(gdb) b start_kernel
Breakpoint 1 at 0xffffffff82973ac0: file init/main.c, line 538.
(gdb) c
Breakpoint 1, start_kernel () at init/main.c:538
538	{
(gdb) list
533	{
534		rest_init();
535	}
536	
537	asmlinkage __visible void __init start_kernel(void)
538	{
539		char *command_line;
540		char *after_dashes;
541	
542		set_task_stack_end_magic(&init_task);