build: split TARGET_PREFER_32_BIT for apps and executables

Introduce two new flags, TARGET_PREFER_32_BIT_APPS and
TARGET_PREFER_32_BIT_EXECUTABLES that individually control
compilation of apps and executables.  If TARGET_PREFER_32_BIT is
set, the build system will automatically set both new flags.

Change-Id: Ibd2165e688675c2884d9e7ddf82d7dd6c28521f3