Pass CXXFLAGS to binutils/gold.

The GOLD linker is written in C++. Pass CT_CFLAGS_FOR_HOST as
CXXFLAGS to configure so that any host specific flags are passed
through.

It feels a bit funny passing CFLAGS as CXXFLAGS, but the PPL and GCC
target rules already do the same.

Signed-off-by: Michael Hope <michael.hope@linaro.org>

 # Blackfin specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_LE

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_SUPPORT_CPU

 ## select ARCH_SUPPORT_TUNE

 ## select ARCH_SUPPORT_FPU

 ##

 ## help The Blackfin architecture