lldb-server had limited support for single-stepping through the lr/sc
atomic sequence. This patch enhances that support for all possible
atomic sequences.
The previous version contained an incorrect regex pattern in the test,
causing the riscv-specific test to run on other platforms. This reland
fixes the regex (see lldb/test/API/riscv/step/TestSoftwareStep.py)
d7e23bef6a