cc: add a flag for skipping core passes

It is used for skipping unnecessary compilation steps when the libc
doesn't need to be compiled (eg. when we do not use a C library).

Signed-off-by: Yann Diorcet <diorcet.yann@gmail.com>
Message-Id: <150eadb0117e697d79aa.1353625025@blackmint>
Patchwork-Id: 201222

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_FLOAT_SW=y

 CT_ARCH_arm=y

 CT_BINUTILS_V_2_19_1a=y

 CT_CC_V_4_3_4=y

 CT_CC_LANG_CXX=y

 CT_DEBUG_gdb=y

 CT_GMP_V_4_3_2=y

 CT_MPFR_V_2_4_2=y