fix: guard SSE flags for arm64 build

[sleepyeye]

When installing simpleimageio via pip on an Apple M1 (arm64) MacBook,
the build failed because the CMake configuration unconditionally enabled
x86-specific SSE compiler flags (-msse4.1 and -mpclmul).

These flags are not supported on non-x86 systems and cause compilation
errors such as:

clang++: error: unsupported option '-msse4.1' for target 'arm64-apple-darwin'
clang++: error: unsupported option '-mpclmul' for target 'arm64-apple-darwin'

This PR updates Core/CMakeLists.txt to skip adding the x86‐specific
compiler flags -msse4.1 and -mpclmul on non-x86 platforms
(e.g. macOS arm64).
These flags caused build failures on Apple Silicon (tested with M1).

Changes

• Detect processor architecture with CMAKE_SYSTEM_PROCESSOR.
• Apply -msse4.1 / -mpclmul only when building for x86_64/AMD64.
• Define FPNG_NO_SSE=1 when building on other architectures.

With this change simpleimageio now builds successfully on macOS arm64.

[pgrit]

Thanks for the PR.
I suppose you are using clang, and not AppleClang, as your default C++ compiler? Because AppleClang ignores those flags and compiles fine.

[sleepyeye]

Yes. My default compiler is Homebrew clang.