AArch64: work around ld64 bug more aggressively.

ld64 currently mishandles internal pointer relocations (i.e.
ARM64_RELOC_UNSIGNED referred to by section & offset rather than symbol). The
existing __cfstring clause was an early discovery and workaround for this, but
the problem is wider and we should avoid such relocations wherever possible for
now.

This code should be reverted to allowing internal relocations as soon as
possible.

PR23437.

llvm-svn: 237621