scripts/xldd: better find sysroot with old gcc

Only starting with 4.4 does gcc have a -print-sysroot option.
For 4.3 or before, we have to play some tricks:
- ask gcc where libc.so is,
(we expect it in ${sysroot}/usr/lib/libc.so)
- trim /usr/lib/libc.so from the result

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from aa2305c5b2a57fbcda9a1b5bb5fb3a476ac49a13)

# powerpc specific configuration file

config ARCH_powerpc

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64 if EXPERIMENTAL

    select ARCH_DEFAULT_32

    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