[LLD][ELF][AArch64] Change the semantics of -z pac-plt.

Summary:
Generate PAC protected plt only when "-z pac-plt" is passed to the
linker. GNU toolchain generates when it is explicitly requested[1].
When pac-plt is requested then set the GNU_PROPERTY_AARCH64_FEATURE_1_PAC
note even when not all function compiled with PAC but issue a warning.
Harmonizing the warning style for BTI/PAC/IBT.
Generate BTI protected PLT if case of "-z force-bti".

[1] https://www.sourceware.org/ml/binutils/2019-03/msg00021.html

Reviewers: peter.smith, espindola, MaskRay, grimar

Reviewed By: peter.smith, MaskRay

Subscribers: tatyana-krasnukha, emaste, arichardson, kristof.beyls, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74537