src/string_bytes.cc - New 64bit platforms. Big endian changes.

Add new 64bit platforms. Mangle ucs2 for big endian.

64bit constants are keyed for x64 platforms only, add PowerPC based
platform constants.

Node's "ucs2" encoding wants LE character data stored in
the Buffer, so we need to reorder on BE platforms.  See
http://nodejs.org/api/buffer.html regarding Node's "ucs2"
encoding specification