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)

 # Super-H specific configuration file

 config ARCH_sh

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     select ARCH_DEFAULT_LE

     help

       The Super-H architecture, as defined by:

         http://www.renesas.com/fmwk.jsp?cnt=superh_family_landing.jsp&fp=/products/mpumcu/superh_family/