toolchain: remove toolchain/Makefile.in and toolchain/gcc/Makefile.in

Re-integrate in gcc-uclibc-4.x.mk things from
toolchain/gcc/Makefile.in that were completely gcc-specific. There was
no reason to pull that when building with other backends than the
internal one.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>