gdb: Make dbxread rely less on global variables

gdb: Make dbxread rely less on global variables

The file dbxread.c, which is responsible for reading stabs information
for multiple file formats, relies heavily on setting and using global
variables over the course of reading symbols.

Future patches aim to make stabs reading more file format independent,
and this patch starts that change by introducing a stabs_context struct,
that will hold all the relevant variables. This context struct is saved
on the registry key inside the objfile being read. Some of those global
variables have been deemed irrelevant:
* dbxread_objfile - Since we're saving in an objfile, this is redundant
* symfile_bfd - It is trivial to get the bfd pointer from the objfile,
  so also unnecessary
* string_table_offset - was never initialized, just used to set a value.
  That usage was substituted by a hardcoded 0
* next_file_string_table_offset - was only used by read_dbx_symtab, so
  it was turned into a local variable there.

As I was moving variables, I also couldn't think of a good reason for
the bincl_list to be a pointer, so it was changed to just be an
std::vector.

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