scripts, cc/gcc: do not fail on existing symlinks or build.log

If the user builds a toolchain over an existing one, so, without removing
CT_PREFIX_DIR, the build fails as the symlinks already exist, as does the
build.log.

This can also happen (for build.log) if the user first ran in download-
or extract-only.

Patch (with no SoB) originally from:
Phil Wilshire <phil.wilshire@overturenetworks.com>

Modified by me as it did not apply cleanly.

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

 # Dummy config file to not use a C library *at all*

 ## depends on BARE_METAL

 ##

 ## help Do not use a C library.

 ## help

 ## help This is usefull if your project is self-contained, does not rely on

 ## help an external C library, and provides all the necessary bits.

 ## help

 ## help Most probably usefull to bootloaders, as they generally don't depend

 ## help on the C library.

 ## help

 ## help If unsure: do *not* choose that, and use another option in the choice.