[AArch64] Add clang command line support for -mharden-sls=

The accepted options to -mharden-sls= are:
* all: enable all mitigations against Straight Line Speculation that are
  implemented.
* none: disable all mitigations against Straight Line Speculation.
* retbr: enable the mitigation against Straight Line Speculation for RET
  and BR instructions.
* blr: enable the mitigation against Straight Line Speculation for BLR
  instructions.

Differential Revision: https://reviews.llvm.org/D81404