build: default to MSVC ABI on Windows

Zig defaults to the GNU ABI on Windows, which produces COFF objects
with invalid COMDAT sections in compiler_rt that the MSVC linker
rejects (LNK1143), and uses GNU conventions like ___chkstk_ms that
are unavailable in the MSVC CRT.

Default to the MSVC ABI when no explicit ABI is requested, following
the same pattern as the existing macOS target override. This ensures
compiler_rt produces valid COFF and the generated code uses
MSVC-compatible symbols. Users can still explicitly request the GNU
ABI via -Dtarget.

Also disable bundling ubsan_rt on Windows (its /exclude-symbols
directives are MSVC-incompatible) and add ntdll and kernel32 as
transitive link dependencies for the static library.