[lldb] use EXT_SUFFIX for python extension

LLDB doesn't use only the python stable ABI, which means loading
it into an incompatible python can cause the process to crash.
_lldb.so should be named with the full EXT_SUFFIX from sysconfig
-- such as _lldb.cpython-39-darwin.so -- so this doesn't happen.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D112972