build: use gcc-provided ar, nm and ranlib where appropriate

Since GCC 4.7, GCC provides its own wrappers around ar, nm and ranlib, which
should be used for builds with link-time optimization. Since GCC 4.9, using them
actually necessary for LTO builds using convenience libraries to succeed.

There are some packages which try to automatically detect if gcc-{ar,nm,ranlib}
exist (one example is my package "fastd" in the package repository, which tries
to use LTO). This breaks because the OpenWrt build system explicitly sets the
binutils versions of these tools.

As it doesn't cause any issues to use gcc-{ar,nm,ranlib} instead of
{ar,nm,ranlib} even without LTO, this patch just makes OpenWrt use the
GCC-provided versions by default, which fixes the build of such packages with
GCC 4.9.

(I know that builds fail though when clang is used with -flto and
gcc-{ar,nm,ranlib}, but as all OpenWrt toolchains are based on GCC, this isn't
a real issue.)

Completely cleaning the tree (or at least `make clean toolchain/clean`) is
necessary to get a consistent state after the binutils plugins support patch and
this one (as trying to use gcc-{ar,nm,ranlib} with a binutils built without
plugin support will definitely lead to a build failure).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43784 3c298f89-4303-0410-b956-a3cf2f4a3e73

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