Integrate cargo-xwin for cross compiling to Windows MSVC targets

[messense]

Closes #830

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[messense]

cc @ravenexp

[messense]

Continuing from rust-cross/cargo-zigbuild#36 (comment).

I think zig dlltool is same as llvm-dlltool and they are not compatible with mingw64's dlltool, thus use zig dlltool for windows gnu targets isn't possible? @ravenexp

[ravenexp]

It depends on what linker zig cc uses for its MinGW targets. I think Zig always links with lld, which uses the Windows import library format, same as LINK.EXE. MinGW-format import libraries are only suitable when x86_64-w64-mingw32-gcc is used as the linker.

[ravenexp]

It depends on what linker zig cc uses for its MinGW targets. I think Zig always links with lld, which uses the Windows import library format, same as LINK.EXE.

I checked this directly just to be sure, and zig cc indeed uses lld-link when compiling for -target x86_64-windows-gnu:

$ zig cc -v -target x86_64-windows-gnu test.c

--skip--
lld-link -ERRORLIMIT:0 -NOLOGO -STACK:16777216 -MACHINE:X64 -OUT:/x/.cache/zig/o/d5879460d6961e986517a114b13aaef2/a.exe -IMPLIB:/x/.cache/zig/o/d5879460d6961e986517a114b13aaef2/test.lib`

This means dlltool from MinGW binutils can not be used with cargo zigbuild and maturin --zig.

Using zig dlltool with zig cc is indeed the only correct option.