scripts: fix finishing the toolchain when download/extract-only is set

In case we only download or extract the sources, do not fail while
finishing the toolchain: the test-suite directory may not exist, so
we can't chmod it.

Also, use safer constructs that won't trigger the 'set -e' in case of
failure (eg.: "[ ... ] && ..." is not safe in case the test fails).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_FLOAT_SW=y

 CT_ARCH_mips=y

 CT_BINUTILS_V_2_19_1a=y

 CT_CC_V_4_3_2=y

 CT_LIBC_none=y

 CT_DEBUG_gdb=y

 CT_GDB_V_6_8a=y

 CT_GMP_V_4_3_2=y

 CT_MPFR_V_2_4_2=y