Enable gold linker for aarch64.

Switches default linker from -fuse-ld-bfd to
-fuse-ld=gold, and enables -Wl,--icf=safe. This
changes reduces /system/lib64/*.so text size
by about 2% for N9.

Change-Id: I0ef2483e1c47c34e63292dad6f6bf532359b733e