Implement {get,record}_sp_limit on Win64

Uses ArbitraryUserPointer area at gs:0x28.
Set main_name "WinMain" on Win64
Add Win64 types
std: Bind write() correctly on Win64
std: Add Win64 support

Some extern blobs are duplicated without "stdcall" abi,
since Win64 does not use any calling convention.
(Giving any abi to them causes llvm producing wrong bytecode.)
Set Win64 target_triple from mingw-w64 name
rt: Make valgrind Win64-compatible