Add an optional offset option to the "add-symbol-file" command

Add an optional offset option to the "add-symbol-file" command

If all sections of a symbol file are loaded with a fixed offset, it
is easier to specify that offset than listing all sections
explicitly.  There is also a similar option for "symbol-file".

gdb/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* symfile.c (add_symbol_file_command, _initialize_symfile): Add
	option "-o" to add-symbol-file-load to add an offset to each
	section's load address.
	* symfile.c (set_objfile_default_section_offset): New function.

gdb/doc/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* gdb.texinfo (Files): Document "add-symbol-file -o offset".

gdb/testsuite/ChangeLog:
2018-06-28  Petr Tesarik  <ptesarik@suse.cz>

	* gdb.base/relocate.exp: Add test for "add-symbol-file -o ".