[LLDB] Fix build on windows caused by LinuxPTraceDefines_arm64sve.h

This patch fixes build on lldb-x64-windows-ninja. The error is caused by
use of two leading underscores.

According to MSVC documentation:
In Microsoft C++, identifiers with two leading underscores are reserved
for compiler implementations.

https://docs.microsoft.com/en-us/cpp/cpp/keywords-cpp?view=vs-2019