Make sure we use the x64 toolchain when cross-compiling from ARM.
Make sure we use the x64 toolchain when cross-compiling from ARM.

The effect of this changeset is to explicitly state that we're building
for x86_64 when cross-compiling from an Apple M1 machine.

This is necessary as CMake will default to building using the ARM64
toolchain if CMAKE_OSX_ARCHITECTURES is unset.

meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk