debug/gdb: fix canadian-cross in case of static cross gdb

Building the cross-gdb shoud be done using the host compiler,
not the native compiler.

Reported-by: Per Arnold Blaasmo <per-arnold.blaasmo@atmel.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/"

reporter_comment="Toolchain for the Raspberry Pi, with hard-float."