crt: Use wcstok_s and _fseeki64 on arm64ec msvcrt.dll

These are available since Windows Vista - so we can rely on them
existing on arm64ec, just like on arm/aarch64.

This fixes linking the wcstok and fseeko64 functions, as the
helper functions provided on x86 aren't built in an arm64ec build
of the mingw-w64-crt.

Signed-off-by: Martin Storsjö <martin@martin.st>