Log Message:
Move the *_amd64 PLIST entries out of the common file and into all of the
platform files except Darwin.  Also add the *_sse2 entries to Darwin.

Note that both *_amd64 and *_sse2 are conditional on x86_64, so this
division of files per platform is easier to do by PLIST tweaks than by
further Makefile conditionals.

This fixes the build of botan under OS X Lion with ABI=64 and should be
a no-op for all other platforms.