[NFC][Thumb] Update frame-chain codegen test to use thumbv6m

The `llvm/test/CodeGen/Thumb/frame-chain.ll`, recently added by D125094,
currently fails when expensive checks are enabled due to a tMOVr
instruction that is only valid from V6 onwards.

The use of the invalid instruction is unrelated to the contents of the
original patch, and continues to be triggered by this test if its
CodeGen changes are reverted, so this patch updates the test to use V6-M
while the issue is not resolved.