Node test simple/test_buffer.js fails on Linux PowerPPC

This commit fixes 3 issues related to endianness assumptions
in node code.

1. StringBytes::Write() needs to convert the platform endian
   16-bit character codes returned by v8::String::Write()
   into little endian form when using 'ucs2' encoding.

2. StringBytes::Encode() needs to convert the little endian
   16-bit character codes stored in 'ucs2' encoded Buffer
   data to platform endian form when calling into
   v8::String::New().

3. StringBytes::Write() needs to select the correct byte of
   character data to use / throw away from what is returned
   by v8::String::Write() when using 'binary' encoding.
   Since the data returned is platform endian, the correct
   byte is different on big endian platforms.

[RTC Defect 44983]