bdwgc does not recognize "uclinux" as a valid OS part of the target
tuple which is used by some arm cortex-M toolchains.
Fixes:
http://autobuild.buildroot.net/results/94f/94fbc1e5afe183e5b071d1e869b2d780025389e2
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
