[arm64] Build code as position independent

Build arm64 kernel as position independent without any relocation for
now. This change is straightforward on arm64 as everything is relative
and GOT usage is already limited.

Disable PIC only for x86_64 until the next commit. Also add a hidden.h
to enforce default hidden on all symbols.

SEC-31
Change-Id: I9ce4fcd33b9eddf817315f1329cff3c9c4842b8c