Add debug info for link-time generated thunks.

Add debug info for method call thunks (currently unused) and
Baker read barrier thunks. Refactor debug info generation
for trampolines and record their sizes; change their names
to start with upper-case letters, so that they can be easily
generated as `#fn_name`.

This improved debug info must be generated by `dex2oat -g`,
the debug info generated by `oatdump --symbolize` remains
the same as before, except for the renamed trampolines and
an adjustment for "code delta", i.e. the Thumb mode bit.

Cortex-A53 erratum 843419 workaround thunks are not covered
by this CL.

Test: Manual; run-test --gdb -Xcompiler-option -g 160, pull
      symbols for gdbclient, break in the introspection
      entrypoint, check that gdb knows the new symbols
      (and disassembles them) and `backtrace` works when
      setting $pc to an address in the thunk.
Bug: 36141117
Change-Id: Id224b72cfa7a0628799c7db65e66e24c8517aabf
diff --git a/compiler/debug/elf_symtab_writer.h b/compiler/debug/elf_symtab_writer.h
index af9f091..abd2699 100644
--- a/compiler/debug/elf_symtab_writer.h
+++ b/compiler/debug/elf_symtab_writer.h
@@ -65,7 +65,7 @@
       continue;  // Add symbol only for the first instance.
     }
     size_t name_offset;
-    if (info.trampoline_name != nullptr) {
+    if (!info.trampoline_name.empty()) {
       name_offset = strtab->Write(info.trampoline_name);
     } else {
       DCHECK(info.dex_file != nullptr);