[ELF][AArch64] Rename --force-bti to -z force-bti and --pac-plt to -z pac-plt

Summary:
The original design used --foo but the upstream complained that ELF only
options should be -z foo. See https://sourceware.org/ml/binutils/2019-04/msg00151.html
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=8bf6d176b0a442a8091d338d4af971591d19922c
made the rename.

Our --force-bti and --pac-plt implement the same functionality, so it
seems wise to be consistent with GNU ld.

Reviewed By: peter.smith

Subscribers: emaste, arichardson, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71327