Merge exec and non-exec segments while merging executable bit.

The bfd and gold linkers create segments like this: r/x, r/w where
the r/x segment covers the start of the ELF file.

lld's segments look like this: r, r/x, r/w where the r segment covers
the start of the ELF file.

So we cannot rely on the location of the r/x to tell where the start
of the ELF is. But we can still rely on the r and r/x mappings being
adjacent. So what we do is when we see an r segment followed by an r/x,
merge the r into the r/x and claim that it is executable. This way,
the minidump writer will continue to see a single executable segment
covering the entire executable.

Testing: "make check" passes when breakpad is compiled with
lld compiled from trunk (requires bug fix from LLVM r303689).

Also patched change into chromium and tested these builds:

$ cat args.gn
is_chrome_branded = true
is_debug = false
is_official_build = true
use_lld = true
allow_posix_link_time_opt = false
is_cfi = false

$ cat args.gn
target_os = "android"
target_cpu = "arm"
is_debug = false
is_official_build = true
is_chrome_branded = true

With both builds breakpad_unittests passes and
chrome/chrome_modern_public_apk create good minidumps after navigating
to chrome://inducebrowsercrashforrealz (checked that minidump contains
stack trace entry for content::HandleDebugURL).

Bug: chromium:716484
Change-Id: Ib6ed3a8420b83acf4a5962843930fb006734cb95
Reviewed-on: https://chromium-review.googlesource.com/513610
Reviewed-by: Primiano Tucci <primiano@chromium.org>

src/client/linux/minidump_writer/linux_dumper.cc

@@ -520,15 +520,20 @@ bool LinuxDumper::EnumerateMappings() {
             name = kLinuxGateLibraryName;
             offset = 0;
           }
-          // Merge adjacent mappings with the same name into one module,
+          // Merge adjacent mappings into one module, assuming they're a single
-          // assuming they're a single library mapped by the dynamic linker
+          // library mapped by the dynamic linker. Do this only if their name
+          // matches and either they have the same +x protection flag, or if the
+          // previous mapping is not executable and the new one is, to handle
+          // lld's output (see crbug.com/716484).
           if (name && !mappings_.empty()) {
             MappingInfo* module = mappings_.back();
             if ((start_addr == module->start_addr + module->size) &&
                 (my_strlen(name) == my_strlen(module->name)) &&
                 (my_strncmp(name, module->name, my_strlen(name)) == 0) &&
-                (exec == module->exec)) {
+                ((exec == module->exec) || (!module->exec && exec))) {
+              module->system_mapping_info.end_addr = end_addr;
               module->size = end_addr - module->start_addr;
+              module->exec |= exec;
               line_reader->PopLine(line_len);
               continue;
             }