MSVC support

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.