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)

# dmalloc debug facility

# depends on ! BACKEND

config DEBUG_dmalloc

    bool

choice

    bool

    prompt "dmalloc version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config DMALLOC_V_5_5_2

    bool

    prompt "5.5.2"

endchoice

config DMALLOC_VERSION

    string

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "5.5.2" if DMALLOC_V_5_5_2