[LLDB] Avoid using InitializeContext for zero-initializing a CONTEXT. NFC.

InitializeContext is useful for allocating a (potentially variable
size) CONTEXT struct in an unaligned byte buffer. In this case, we
already have a fixed size CONTEXT we want to initialize, and we only
used this as a very roundabout way of zero initializing it.

Instead just memset the CONTEXT we have, and set the ContextFlags field
manually.

This matches how it is done in NativeRegisterContextWindows_*.cpp.

This also makes LLDB run successfully in Wine (for a trivial tested
case at least), as Wine hasn't implemented the InitializeContext
function.

Differential Revision: https://reviews.llvm.org/D70742