Adjust the frame setup code for Windows ARM64 to attempt to align
pair-wise spills to 16-byte boundaries. This enables us to properly emit
the spills for custom clang calling convensions such as preserve most
which spills r9-r15 which are normally nonvolatile registers. Even when
using the ARM64EC opcodes for the unwinding, we cannot represent the
spill if it is unaligned.
2d8563f4b8