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)

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORT_CPU

 ##

 ## help The m68k architecture