executable_prefer_symlink: add makefile for creating symlink to prefer version

For a 64bit target, we can have support for both 32bit and 64bit version
binary(like app_process, dalvikvm), and there are some commands like am/pm
will call these commands to implement it's function.

In some targets it will prefer to use the 32bit version, and in other targets
it will prefer to use the 64bit version. So this change is to add the common
work for creating the symlink to the prefer version binary,and avoid the
change on the caller commands side like am/pm.

In this change, it uses TARGET_PREFER_32_BIT_APPS to decide linking to
32bit version or 64bit version.

Change-Id: I96f041c0e0ededd953feead30ec63ad63e945a6f
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>