gcc47: Restore cross-build capability

gcc47: Restore cross-build capability

The internal specs of gcc47 featured an hardcoded standard include
directory which prevented cross-target builds.  The same mistake also
caused the "crosstools" compiler in a native build to look at the host
system includes instead of the world_{TARGET}/usr/includes directory.
As a result, the cross-build would break as soon as GCC47 was used to
build its own libraries.

A stage-specific prefix was added to the internal specs directory
definition that restored cross-target building capability.