kernel.bbclass: Configuration for environment with HOSTCXX

When compiling xilinx-zynq board linux-kernel-dev(v5.8) if
"GCC_PLUGINS=y", The following error will appear:

"HOSTCXX -fPIC scripts/gcc-plugins/arm_ssp_per_task_plugin.o
fatal error: gmp.h: No such file or directory"

the GCC_PLUGINS depend on return result of gcc-plugin.sh execution
however in gcc-plugin.sh use HOSTCC to detect the feature of GNU
extension of gcc, this will result that HOSTCC can compile the file
successfully, but HOSTCXX is used in the actual compilation process.

Signed-off-by: Zhang Qiang <qiang.zhang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>