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>
(transplanted from 1ebc2248cc60230cd53ff94ae8f8f1e3261461a3)

# Bare metal config options

config KERNEL_bare_metal

    select BARE_METAL

    help

      'Bare metal' refers to those programs that run without any kernel.

      You probably want to say 'y' here if you plan to use your compiler

      to build bootloaders. It is not yet suitable to build Linux kernels,

      though, because the APCI stuff relies on the target C library headers

      being available?!?!...