[hwasan] Add default "/" prefix.

[eugenis]

Summary

Add a default "/" prefix to the symbol search path in the
symbolization script. Without this, the binary itself is not considered
a valid source of symbol info.

[hctim]

compiler-rt/lib/hwasan/scripts/hwasan_symbolize

199–202

this branch is now dead.

[eugenis]

address comment

compiler-rt/lib/hwasan/scripts/hwasan_symbolize

199–202

That's a very good point.

compiler-rt/lib/hwasan/scripts/hwasan_symbolize

 parser.add_argument('--ignore-tags', action='store_true')
 parser.add_argument('--symbols', action='append')
 parser.add_argument('--source', action='append')
 parser.add_argument('--symbolizer')
 parser.add_argument('args', nargs=argparse.REMAINDER)
 args = parser.parse_args()
 
 # Unstripped binaries location.
-binary_prefixes = args.symbols or []
+binary_prefixes = args.symbols or ['/']
 if not binary_prefixes:
   if 'ANDROID_PRODUCT_OUT' in os.environ:
     product_out = os.path.join(os.environ['ANDROID_PRODUCT_OUT'], 'symbols')
     binary_prefixes.append(product_out)
 
 for p in binary_prefixes:
   if not os.path.isdir(p):
     print >>sys.stderr, "Symbols path does not exist or is not a directory:", p
     sys.exit(1)
 
 # Source location.
 paths_to_cut = args.source or []