toolchain/gcc: add elf2flt to dependencies

Add host-elf2flt to the gcc common dependencies so we get it built after
binutils but before gcc/uclibc since it's required for all packages and
in some uClibc configuration scenarios.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>