gdb: Split print_symbol_info into two parts

gdb: Split print_symbol_info into two parts

Split the function print_symbol_info into two parts, the new worker
core returns a string, which print_symbol_info then prints.  This will
be useful in a later commit when some new MI commands will be added
which will use the worker core to fill some MI output fields.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* symtab.c (symbol_to_info_string): New function, most content
	moved from print_symbol_info, but updated to return a std::string.
	(print_symbol_info): Update to use symbol_to_info_string and print
	returned string.
	* symtab.h (symbol_to_info_string): Declare new function.

Change-Id: I6454ce43cacb61d32fbadb9e3655e70823085777