Add support for compiling Windows ARM64EC

[MugundanMCW]

Description of changes:

• ARM64EC (Emulation Compatible) is an application binary interface (ABI) for applications running on ARM-based Windows 11 devices. Code built for ARM64EC can interoperate with x64 code running under emulation, leveraging the native performance of ARM hardware.
• This PR introduces support for building the aws-checksums library for Windows ARM64EC. It adds conditional checks to exclude non-ARM64 code paths, ensuring successful compilation and compatibility with the ARM64EC architecture.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.