Bug 962488 - Update double-conversion for proper AArch64 support. r=froydnj

Fix imported from upstream repository:
https://code.google.com/p/double-conversion/source/detail?r=4e24bb31bcc76d6d218f3056b4c24a109d367561
Bug 963024 - AArch64 support for XPCOM. r=froydnj