toolchain: keep the initial gcc around for later uclibc rebuilds

toolchain: keep the initial gcc around for later uclibc rebuilds

testing uclibc changes is tricky because the final gcc tends to miscompile
uclibc code or barf up internal compiler errors.
install binutils into $(TOOLCHAIN_DIR)/initial (without changing the configure
prefix) and copy it from there to $(TOOLCHAIN_DIR)/ so that the initial gcc
can be put into $(PATH) for the uclibc build, even if the final gcc
is already installed.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32553 3c298f89-4303-0410-b956-a3cf2f4a3e73