Introduce common/symbol.h

Introduce common/symbol.h

This introduces common/symbol.h.  This file declares a function that
the shared code can use and that the clients must implement.  It also
changes some shared code to use these functions.

gdb/ChangeLog:

	* common/symbol.h: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add common/symbol.h.
	* minsyms.c (find_minimal_symbol_address): New function.
	* common/agent.c: Include common/symbol.h.
	[!GDBSERVER]: Don't include objfiles.h.
	(agent_look_up_symbols): Use find_minimal_symbol_address.

gdb/gdbserver/ChangeLog:

	* symbol.c: New file.
	* Makefile.in (SFILES): Add symbol.c.
	(OBS): Add symbol.o.