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>