MSVC support

[LukasKastern]

Building with zig build -Dtarget=x86-64-windows-msvc fails tragically with various std conflicts:

error: sub-compilation of libcxxabi failed
    :1:1: note: argument unused during compilation: '-nostdinc++'
    :1:1: note: argument unused during compilation: '-nostdinc++'
    C:\Zig\lib\libcxxabi\src/stdlib_new_delete.cpp:57:32: note: no member named 'get_new_handler' in namespace 'std'
    std::new_handler nh = std::get_new_handler();
                          ~~~~~^
    C:\Zig\lib\libcxxabi\src/stdlib_new_delete.cpp:150:32: note: no member named 'get_new_handler' in namespace 'std'
    std::new_handler nh = std::get_new_handler();

    :1:1: note: too many errors emitted, stopping now
    :1:1: note: argument unused during compilation: '-nostdinc++'
    :1:1: note: argument unused during compilation: '-nostdinc++'
    C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.38.33130\include/vcruntime_typeinfo.h:137:13: note: target of using declaration conflicts with declaration already in scope
    using ::type_info;
....

MSVC is the tested platform for boringssl on windows so should get this working eventually.