linux: Move the struct stat{64} to struct_stat.h

The common definitions are moved to a Linux generic stat.h while the
struct stat{64} definition are moved to a arch-specific struct_stat.h
header.

Checked with a build for all affected ABIs. I also checked on x86_64,
i686, powerpc, powerpc64le, sparcv9, sparc64, s390, and s390x.

Reviewed-by: Lukasz Majewski <lukma@denx.de>