[lldb/test] Use inline assembly for instruction counting tests

We have a test which checks that instruction-step really steps one
instruction, but the way it checks this makes it very susceptible to
codegen changes. This rewrites the test inferior to use inline assembly,
which guarantees a known sequence of instructions that the test can
check. This does mean we have to write separate assembly for each
architecture, but that is no better than having architecture-specific
assertions, which the test was already starting to accumulate.