[AArch64] only indicate CFI on Windows if we emitted CFI

Summary:
Otherwise, we emit directives for CFI without any actual CFI opcodes to
go with them, which causes tools to malfunction.  The technique is
similar to what the x86 backend already does.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40876

Patch by: froydnj (Nathan Froyd)

Reviewers: mstorsjo, eli.friedman, rnk, mgrang, ssijaric

Reviewed By: rnk

Subscribers: javed.absar, kristof.beyls, llvm-commits, dmajor

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61960

llvm-svn: 360816