x86_64 vs amd64 arch name for Eclipse

[ken]

  Steph,
  It reduces large part of boring porting work, and I prefer to use x86_64 rather than amd64 internally.  However, if users need to use the word "x86_64" explicitly for compilation, it may not be a preferred choice from port/src tree consistency viewpoint.

  In short, one to "x86_64" if it is handled internally and explanation for amd64 vs x86_64 is given.