external toolchain: use LANG=C when calling gcc -v

Lionel Landwerlin <lionel.landwerlin@openwide.fr> reported that using
the external toolchain support when LANG=fr_FR.UTF-8 doesn't work,
since the messages printed by gcc -v are translated in another
language, defeating the grep ^Configured test.

Therefore, as per Lionel suggestion, we force LANG=C when calling
$(TARGET_CC) -v.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>