[ELF] Add llvm-readelf to the lit tool substitutions

[arichardson]

Summary

Some of the tests invoke llvm-readelf. This currently appears to work
probably because the LLVM binary directory is included in $PATH. However,
this is quite fragile so let's just make lit expand the full path.

[ruiu]

LGTM

test/lit.cfg.py

 config.test_source_root = os.path.dirname(__file__)
 
 config.test_exec_root = os.path.join(config.lld_obj_root, 'test')
 
 llvm_config.use_default_substitutions()
 llvm_config.use_lld()
 
 tool_patterns = [
-    'llc', 'llvm-as', 'llvm-mc', 'llvm-nm',
-    'llvm-objdump', 'llvm-pdbutil', 'llvm-readobj', 'obj2yaml', 'yaml2obj']
+    'llc', 'llvm-as', 'llvm-mc', 'llvm-nm', 'llvm-objdump', 'llvm-pdbutil',
+    'llvm-readelf', 'llvm-readobj', 'obj2yaml', 'yaml2obj']
 
 llvm_config.add_tool_substitutions(tool_patterns)
 
 # When running under valgrind, we mangle '-vg' onto the end of the triple so we
 # can check it with XFAIL and XTARGET.
 if lit_config.useValgrind:
     config.target_triple += '-vg'