Re-enable gold linker for aarch64.

Turn back on ld.gold and W-l,--icf=safe for
aarch64, now that the prebuilt ld.gold has been updated
with support for reloc 311/312 (fixed upstream, see
https://sourceware.org/bugzilla/show_bug.cgi?id=19042)

Bug: 25642296
Bug: 26153840

Change-Id: Idceb357a48d9da4eec38ab8f2103245d500622ae