[AArch64][SME] Set fn attributes correctly on __arm_tpidr2_save call.

Changing the way the attribute is created changes the way the attribute
is added. Now it seems to be correctly added as a function attribute, whereas
before it was not. This led ISel ignoring attributes and setting up a lazy-save
buffer for the call to __arm_tpidr2_save.

This patch also marks the intrinsic call as 'preserves_za' which together with the
above prevents the code-generator from setting up a lazy-save buffer.

Reviewed By: kmclaughlin

Differential Revision: https://reviews.llvm.org/D136342