This extends the target selection implemented in dad6ec3aa6 to support
Windows ARM64 platforms. OpenSSL support for VC-WIN64-ARM target first
appeared in 1.1.1 and is present in all currently supported (3.x)
branches.
As a side effect, ARM64 Windows builds will get 16-byte alignment along
with the rest of non-x86 platforms. This is safe, as malloc on 64-bit
Windows guarantees the fundamental alignment of allocations, 16 bytes.