Use --gc-sections during link

All common functions are being built into all binary images,
whether or not they are actually used. This change enables the
use of -ffunction-sections, -fdata-sections and --gc-sections
in the compiler and linker to remove unused code and data from
the images.

Change-Id: Ia9f78c01054ac4fa15d145af38b88a0d6fb7d409