[CMake] Simplify CPU architecture detection logic (#54637)
Summary:
CMAKE_SYSTEM_PROCESSOR set to x86_64(on Linux) or AMD64 (https://github.com/pytorch/pytorch/commit/5ec224496b1e42bd24d17ec4b75e4e4d3ae71ef2)(on Windows) indicates build is running on x86_64 architecture, while `CMAKE_SYSTEM_PROCESSOR` set to aarch64 or arm64 means we running on ARMv8+ architecture.
Delete `i[3-6]86` pattern as 32-bit builds are no longer supported
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54637
Reviewed By: ezyang
Differential Revision: D27311897
Pulled By: malfet
fbshipit-source-id: 26989fc9b54a96d70c768ab03ca4528506ee7808
