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

If the main file is relocated at runtime, all symbols are offset by
a fixed amount.  Let the user specify this offset when loading a
symbol file.

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

	* symfile.c (symbol_file_command, symbol_file_add_main_1)
	(_initialize_symfile): Add option "-o" to symbol-file to add an
	offset to each section of the symbol file.

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

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

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

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