gdb: Move dbx_end_psymtab to stabsread, and rename to stabs_end_psymtab

gdb: Move dbx_end_psymtab to stabsread, and rename to stabs_end_psymtab

This function is used by multiple stabs readers (even if not all), and
the comment in stabsread.h even acknowledges it. I believe that the
comment is incorrect in saying that the function should be in dbxread
because not everyone uses it. If any one reader other than dbx uses
it, the function should be in stabsread, in my opinion.

This commit makes also renames the function to stabs_end_psymtab since,
again, this is not specific to dbx/aout format.

struct symloc had to be moved because stabs_end_psymtab dereferences
symloc objects, so stabsread.c must be aware of the full struct.

Approved-By: Tom Tromey <tom@tromey.com>