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)

Reverts llvm/llvm-project#127505 because
`riscv/step/TestSoftwareStep.py` is failing on the bots.

lldb-server had limited support for single-stepping through the lr/sc
atomic sequence. This patch enhances that support for all possible
atomic sequences.

Introduce mips64 support to match the legacy FreeBSD plugin. Similarly
to the legacy plugin, the code does not support FPU registers at the
moment.  The support for them will be submitted separately as it
requires changes to the register context shared by both plugins.

This also includes software single-stepping support that is moved from
the Linux plugin into a common Utility class.  The FreeBSD code also
starts explicitly ignoring EINVAL from PT_CLEARSTEP since this is easier
to implement than checking whether hardware single-stepping were used.

Differential Revision: https://reviews.llvm.org/D95802