[AArch64] Avoid incompatibility between SLSBLR mitigation and BTI codegen.

A "BTI c" instruction only allows jumping/calling to using a BLR* instruction.
However, the SLSBLR mitigation changes a BLR to a BR to implement the
function call. Therefore, a "BTI c" check that passed before could
trigger after the BLR->BL change done by the SLSBLR mitigation.
However, if the register used in BR is X16 or X17, this trigger will not
fire (see ArmARM for further details).

Therefore, this patch simply changes the function stubs for the SLSBLR
mitigation from
__llvm_slsblr_thunk_x<N>:
    br x<N>
    SpeculationBarrier
to
__llvm_slsblr_thunk_x<N>:
    mov x16, x<N>
    br  x16
    SpeculationBarrier

Differential Revision: https://reviews.llvm.org/D81405