Add .dwarf2_addr_size directive to gas

Add .dwarf2_addr_size directive to gas

gas assumes that the address size used for DWARF debug info (and thus the
line tables it generates from .loc directives) will always match the ELF
class. However LLVM generates debug info that matches the actual address
size (32 bit on x86-64 NaCl), which results in a mismatch between LLVM's
debug info and gas' line tables.

This CL adds a directive to gas to change the address size.

R=mcgrathr@chromium.org
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3966

Committed: https://git.chromium.org/gitweb?p=native_client/nacl-binutils.git;a=commit;h=f009fc843ba7b9028f394725961bf899a172f45b

[Roland McGrath, 2014-11-25 22:22:06]

lgtm

[Derek Schuff, 2014-11-25 22:24:51]

Committed patchset #1 (id:1) manually as
f009fc843ba7b9028f394725961bf899a172f45b (presubmit successful).