gdb: Add functions to make loading from dynamically positioned targets easier

gdb: Add functions to make loading from dynamically positioned targets easier

Many targets, such as EFI, load GRUB at addresses that are determined at
runtime. So the load addresses in kernel.exec will almost certainly be
wrong. Given the address of the start of the text segment, these
functions will tell GDB to load the symbols at the proper locations. It
is left up to the user to determine how to get the text address of the
loaded GRUB image.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>