cc/gcc: Apply CT_CC_GCC_DISABLE_PCH to do_cc_core.

Otherwise, users have to input --disable-libstdcxx-pch option
when building bare-metal CANADIAN C++ compiler.

Reviewed-by: Michael Hope
Signed-off-by: Zhenqiang Chen <zhenqiang.chen@linaro.org>
(transplanted from e3e1c9d45bddfbfb433ee9d583faf42fa31f50c0)

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =