[Support] [BLAKE3] Fix compilation with CMAKE_OSX_ARCHITECTURES

With CMake, one can build for multiple macOS architectures
at the same time by setting CMAKE_OSX_ARCHITECTURES to multiple
architectures (avoiding needing to do two separate builds and
gluing the binaries together after the build).

In this case, while targeting x86_64 and arm64, neither IS_X64
nor IS_ARM64 is set, while compilation of the individual source
files will hit those cases (in either architecture mode).

Therefore, if we on the CMake level decide not to include the
architecture specific SIMD implementation files, also tell the
source this explicitly by passing the defines indicating that we
don't expect to use them.

Such a build clearly is less ideal than explicitly targeting one
architecture at a time if it won't include all the SIMD optimizations,
but that's a tradeoff that is up to the one deciding to do such an
universal build.

This also fixes builds for i386. The blake3 source code automatically
enables the SIMD implementations when building for i386, but we don't
provide the sources for that build configuration.

Differential Revision: https://reviews.llvm.org/D122884