This adds a new `mlir_arm_runner_utils` library that contains utils
specific to Arm/AArch64. This is for use in MLIR integration tests.
This initial patch adds `setArmVLBits()` and `setArmSVLBits()`. This
allows changing vector length or streaming vector length at runtime (or
setting it to a known minimum, i.e. 128-bits).
