configure: install in a sub-dir of --libdir and --docdir

In case the user specifies the LIBDIR with --libdir , or DOCDIR with
--docdir, install our support files in a sub-dir, so as not to mangle
existing (potentially system) files/dirs.
(transplanted from 991fc21dd3cfda7d6448388b3224821f6937f15e)

# powerpc specific configuration file

config ARCH_powerpc

    select ARCH_USE_MMU

    select ARCH_SUPPORT_ABI

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The PowerPC architecture, as defined by:

        http://www.ibm.com/developerworks/eserver/articles/archguide.html