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>
(transplanted from 9cd87d279a99bd32a364f12d4b2d6d37e1d9248b)

scripts/build/binutils/binutils.sh

@@ -64,6 +64,7 @@ do_binutils() {
 
     CT_DoExecLog CFG                                            \
     CFLAGS="${CT_CFLAGS_FOR_HOST}"                              \
+    CXXFLAGS="${CT_CFLAGS_FOR_HOST}"                            \
     "${CT_SRC_DIR}/binutils-${CT_BINUTILS_VERSION}/configure"   \
         --build=${CT_BUILD}                                     \
         --host=${CT_HOST}                                       \