* read.c (potable): Add weakref.
(s_weakref): New.
* read.h (s_weakref): Declare.
* struc-symbol.h (struct symbol): Add sy_weakrefr and sy_weakrefd.
* symbols.c (colon): Clear weakrefr.
(symbol_find_exact): Rename to, and reimplement in terms of...
(symbol_find_exact_noref): ... new function.
(symbol_find): Likewise...
(symbol_find_noref): ... ditto.
(resolve_symbol_value): Resolve weakrefr without setting their
values.
(S_SET_WEAK): Call hook.
(S_GET_VALUE): Follow weakref link.
(S_SET_VALUE): Clear weakrefr.
(S_IS_WEAK): Follow weakref link.
(S_IS_WEAKREFR, S_SET_WEAKREFR, S_CLEAR_WEAKREFR): New.
(S_IS_WEAKREFD, S_SET_WEAKREFD, S_CLEAR_WEAKREFD): New.
(symbol_set_value_expression, symbol_set_frag): Clear weakrefr.
(symbol_mark_used): Follow weakref link.
(print_symbol_value_1): Print weak, weakrefr and weakrefd.
* symbols.h (symbol_find_noref, symbol_find_exact_noref): Declare.
(S_IS_WEAKREFR, S_SET_WEAKREFR, S_CLEAR_WEAKREFR): Declare.
(S_IS_WEAKREFD, S_SET_WEAKREFD, S_CLEAR_WEAKREFD): Declare.
* write.c (adust_reloc_syms): Follow weakref link.  Do not
complain if target is undefined.
(write_object_file): Likewise.  Remove weakrefr symbols.  Drop
unreferenced weakrefd symbols.
* config/obj-coff.c (obj_frob_symbol): Do not force WEAKREFD
symbols EXTERNAL.
(pecoff_obj_set_weak_hook, pecoff_obj_clear_weak_hook): New.
* config/obj-coff.h (obj_set_weak_hook, obj_clear_weak_hook): Define.
* doc/as.texinfo: Document weakref.
* doc/internals.texi: Document new struct members, internal
functions and hooks.
gas/testsuite/ChangeLog:
* gas/all/weakref1.s, gas/all/weakref1.d: New test.
* gas/all/weakref1g.d, gas/all/weakref1l.d: New tests.
* gas/all/weakref1u.d, gas/all/weakref1w.d: New tests.
* gas/all/weakref2.s, gas/all/weakref3.s: New tests.
* gas/all/gas.exp: Run new tests.