target/i386: emulate: indirect access to CRs

Prepare to have on-demand fetch of registers from the backend during
faults.

For x86_64 macOS, copy the function there too.

Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Link: https://lore.kernel.org/r/20260324151323.74473-9-mohamed@unpredictable.fr
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>