windows: add RtlCaptureContext, RtlLookupFunctionEntry, RtlVirtualUnwind and supporting types windows: fix alignment of CONTEXT structs to match winnt.h as required by RtlCaptureContext (fxsave instr) windows aarch64: fix __chkstk being defined twice if libc is not linked on msvc Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>