[lldb] Fix data race when interacting with python scripts

This patch should fix some data races when a python script (i.e. a
Scripted Process) has a nested call to another python script (i.e. a
OperatingSystem Plugin), which can cause concurrent writes to the python
lock count.

This patch also fixes a data race happening when resetting the operating
system unique pointer.

To address these issues, both accesses is guarded by a mutex.

rdar://109413039

Differential Revision: https://reviews.llvm.org/D154271



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>