Enable gold linker for aarch64.

[Second attempt, this time with updated mac prebuilt]

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: I587075aae9d70cb6b16e55dc9cd1052580ac2626