[ARM] Mitigate the cve-2021-35465 security vulnurability.

[ARM] Mitigate the cve-2021-35465 security vulnurability.

Recently a vulnerability issue is found in the implementation of VLLDM
instruction in the Arm Cortex-M33, Cortex-M35P and Cortex-M55. If the
VLLDM instruction is abandoned due to an exception when it is partially
completed, it is possible for subsequent non-secure handler to access
and modify the partial restored register values. This vulnerability is
identified as CVE-2021-35465.

The mitigation sequence varies between v8-m and v8.1-m as follows:

v8-m.main
---------
mrs        r5, control
tst        r5, #8       /* CONTROL_S.SFPA */
it         ne
.inst.w    0xeeb00a40   /* vmovne s0, s0 */
1:
vlldm      sp           /* Lazy restore of d0-d16 and FPSCR. */

v8.1-m.main
-----------
vscclrm    {vpr}        /* Clear VPR. */
vlldm      sp           /* Lazy restore of d0-d16 and FPSCR. */

More details on
developer.arm.com/support/arm-security-updates/vlldm-instruction-security-vulnerability

Differential Revision: https://reviews.llvm.org/D109157