[Supersize] Add preliminary string literal extraction under LLD.

Previously Supersize archive for binaries linked with LLD (no LTO) did
not support string literals, and required '--no-string-literals' to
avoid triggering assert. This CL fixes this this by adding preliminary
support for merged strings. Wrinkles:
- LLD .map files does not discern string literals from data literals
  (and testing align=1 seems insufficient), so we count everything as
  string literals for now.
- llvm-readobj (AKA llvm-readelf)'s dump for .a files is missing
  'File: libray.a(file.o)' output. For now we work around this by
  using regular readelf instead.

Bug: 723798
Change-Id: Ic0c9a48dfd9ab157ed19471127a5216ee98b8d65
Reviewed-on: https://chromium-review.googlesource.com/777441
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517752}

tools/binary_size/libsupersize/archive.py

@@ -653,8 +653,9 @@ def CreateSizeInfo(map_path, elf_path, tool_prefix, output_directory,
     bulk_analyzer.AnalyzePaths(missed_object_paths)
     bulk_analyzer.SortPaths()
     if track_string_literals:
-      merge_string_syms = [
-          s for s in raw_symbols if s.full_name == '** merge strings']
+      merge_string_syms = [s for s in raw_symbols if
+                           s.full_name == '** merge strings' or
+                           s.full_name == '** lld merge strings']
       # More likely for there to be a bug in supersize than an ELF to not have a
       # single string literal.
       assert merge_string_syms

tools/binary_size/libsupersize/linker_map_parser.py

@@ -330,7 +330,9 @@ class MapFileParserLld(object):
           # merged data. Feature request is filed under:
           # https://bugs.llvm.org/show_bug.cgi?id=35248
           if cur_obj == '<internal>':
-            sym_maker.cur_sym.full_name = '** lld merge section'
+            # Treat all literals as stirng literals.
+            # FIXME(huangs): Refine this. Checking align == 1 is insufficient.
+            sym_maker.cur_sym.full_name = '** lld merge strings'
           else:
             sym_maker.cur_sym.object_path = cur_obj
 

tools/binary_size/libsupersize/path_util.py

@@ -157,4 +157,8 @@ def GetObjDumpPath(tool_prefix):
 
 
 def GetReadElfPath(tool_prefix):
+  # Work-around for llvm-readobj bug where 'File: ...' info is not printed:
+  # https://bugs.llvm.org/show_bug.cgi?id=35351
+  if tool_prefix[-5:] == 'llvm-':
+    return 'readelf'
   return tool_prefix + 'readelf'