Basic handling of CIE version 4 in dwarf reading

CIE looks like it's been emitted by clang since ~May 2015 [1]. This
means that we didn't have any CFI because this parse aborted, which
meant that all stack walks reverted to stack scanning. Allow expected
values for address size and segment descriptor size through so that
dump_syms can generate at least somewhat reasonable data.

[1]: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150518/277292.html

R=mark@chromium.org
BUG=chromium:627529

Change-Id: I6dc92f51c4afd25c2adff92c09ccb8bb03bf9112
Reviewed-on: https://chromium-review.googlesource.com/406012
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/dwarf/cfi_assembler.h

@@ -138,7 +138,9 @@ class CFISection: public Section {
                         unsigned return_address_register,
                         uint8_t version = 3,
                         const string &augmentation = "",
-                        bool dwarf64 = false);
+                        bool dwarf64 = false,
+                        uint8_t address_size = 8,
+                        uint8_t segment_size = 0);
 
   // Append a Frame Description Entry header to this section with the
   // given values. If dwarf64 is true, use the 64-bit DWARF initial