This patch handles x86_32 and x86_64 separately since this is the SPEC
value directly supplied to the package build tool. This way, we achieve
that a CROSS_DEV_PREFIX is defined for each supported argument.
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
