[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.

compiler-rt/lib/hwasan/scripts/hwasan_symbolize

 args = parser.parse_args()
 
 # Unstripped binaries location.
 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)
+  binary_prefixes.append('/')
 
 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 []