package/gcc: fix typo for CFLAGS_FOR_TARGET

CFLAGS_FOR_TARGET is initialized with GCC_COMMON_TARGET_CXXFLAGS.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>