[AArch64][SME] Fix lowering of llvm.aarch64.get.pstatesm()

A thread may not have access to SME or TPIDR2_EL0, so in order to
safely query PSTATE.SM in a streaming-compatible function, the
code should call `__arm_sme_state()`, as described in the ABI:

  https://github.com/ARM-software/abi-aa/pull/123/commits/c2bb09c4d4ee60a5787baf1ccc7e92e67e4240b7

This means that the value of pstate.sm is:
* 0 if the function is non-streaming.
* 1 if the function has `arm_streaming` or `arm_locally_streaming`.
* evaluated at runtime by a call to __arm_sme_state() otherwise.

This patch also adds a calling convention for calls to SME support routines.

At some point we can remove the need for the llvm.aarch64.get.pstatesm() intrinsic
and use function calls (with the corresponding cc) directly instead.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D131571