Reducing memory usage

[Daniel Berlin <dan at cgsoftware dot com>]

For starters, we don't try to not duplicate type info when reading dwarf2.
I've fixed this.
Doing this takes gdb, with all of it's symbols read in, from this:
Statistics for '/usr/local/gdb/src/gdb/gdb':
  Number of "stab" symbols read: 290212
  Number of "minimal" symbols read: 10600
  Number of "partial" symbols read: 160442
  Number of "full" symbols read: 237611
  Number of "types" defined: 129044
  Space used by a.out string tables: 974998
  Total memory used for psymbol obstack: 8017943
  Total memory used for psymbol cache: 948776
  Total memory used for symbol obstack: 24542604
  Total memory used for type obstack: 19411848

to this:

Statistics for '/usr/local/gdb/src/gdb/./gdb':
  Number of "stab" symbols read: 290212
  Number of "minimal" symbols read: 10600
  Number of "partial" symbols read: 160442
  Number of "full" symbols read: 155649
  Number of "types" defined: 105366
  Space used by a.out string tables: 974998
  Total memory used for psymbol obstack: 8017943
  Total memory used for psymbol cache: 948776
  Total memory used for symbol obstack: 18510936
  Total memory used for type obstack: 12585708

A savings of about 13 meg (and in percentages, we save 45% of the type
obstack, 25% of the symbol stack).

This only helps for dwarf2, stabs and whatnot were already doing something
similar.

I used the hashtable implementation libiberty now provides, rather than
introduce yet another gdb hashtable type.

Not too shabby, methinks.

Anyone want to test the patches for me, while i work on some more memory
reduction.
--Dan