[AArch64] Make speculation-hardening-sls.ll x16 test more robust

As suggested in D110830, this copies the Arm backend method of testing
function calls through specific registers, using inline assembly to
force the variable into x16 to check that the __llvm_slsblr_thunk calls
do not use a register that may be clobbered by the linker.

Differential Revision: https://reviews.llvm.org/D111056