[mach-o] Add support for arm64 (AAarch64)

Most of the changes are in the new file ArchHandler_arm64.cpp.  But a few
things had to be fixed to support 16KB pages (instead of 4KB) which iOS arm64
requires.  In addition the StubInfo struct had to be expanded because
arm64 uses two instruction (ADRP/LDR) to load a global which requires two
relocations.  The other mach-o arches just needed one relocation.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217469 91177308-0d34-0410-b5e6-96231b3b80d8