Implement SMULWB, SMULWT, SMLAWB, SMLAWT (AArch32) (#953)

* Implement SMULWB, SMULWT, SMLAWB, SMLAWT, and add tests for some multiply instructions

* Improve test descriptions

* Rename SMULH to SMUL__

Co-authored-by: jduncanator <1518948+jduncanator@users.noreply.github.com>