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)

 # Super-H specific configuration file

 choice

     bool

     prompt "Variant"

 config ARCH_SH_SH3

     bool

     prompt "sh3"

 config ARCH_SH_SH4

     bool

     prompt "sh4"

 config ARCH_SH_SH4A

     bool

     prompt "sh4a"

 endchoice

 config ARCH_SH_VARIANT

     string

     default "sh3"   if ARCH_SH_SH3

     default "sh4"   if ARCH_SH_SH4

     default "sh4a"  if ARCH_SH_SH4A