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)

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/11_all_gcc-netbsd-symbolic.patch

 http://bugs.gentoo.org/122698

 diff -durN gcc-4.3.1.orig/gcc/config/netbsd-elf.h gcc-4.3.1/gcc/config/netbsd-elf.h

 --- gcc-4.3.1.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

 +++ gcc-4.3.1/gcc/config/netbsd-elf.h	2008-06-10 14:57:48.000000000 +0200

 @@ -82,6 +82,7 @@

  #define NETBSD_LINK_SPEC_ELF \

    "%{assert*} %{R*} %{rpath*} \

     %{shared:-shared} \

 +   %{symbolic:-Bsymbolic} \

     %{!shared: \

       -dc -dp \

       %{!nostdlib: \