Fix swt.jar for linux-x86_64.

Fix swt.jar for linux-x86_64.

This moves prebuilt/linux-x86_64/swt into prebuilt/linux-x86/swt_64. The makefiles
always build using linux-x86 so the linux-x86_64 prebuild was never getting built
and it had no makefiles. Now the new prebuilt when invoked using linux-x86 generates
both out/.../swt.jar and out/.../swt_64.jar. This is then picked up by development.git
build/sdk-linux.atree to generate the correct final SDK files.

This depends on a CL from development.git:

  http://android-git.corp.google.com/g/5179