gdb: split iterate_over_symtabs into for_each_symtab and find_symtab

Same rationale as the previous patches.

For the moment, find_symtab is only needed internally in symtab.c, so
keep it static there.  Note that the interaction with
objfile.map_symtabs_matching_filename gets cleaner in a subsequent
patch.

for_each_symtab is implemented using find_symtab, because the iteration
behavior is not completely trivial.

find_symtab_callback_ftype is in the header file, because it is used
from another source file in the next patch.

Change-Id: I6ab8342151eb735327fc2e7935e7a65cede5e1dd
Approved-By: Andrew Burgess <aburgess@redhat.com>