debug/gdb: don;t install sample gdbinit for old versions

Only starting with gdb-7 does installing the gdbinit sample makes senses.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Julien DUSSER"

reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

reporter_comment="Soft-float for the 405 PowerPC"