arc: Make sure that libgcc doesn't get included when it doesn't exist yet

For ARC, libgcc is always included, even when -nostdlib is given. This is
related to some small pieces of code that are not always generated by the
compiler; a call to libgcc is used in those cases instead.

During the initial stages of building the toolchain, this is a problem, as
libgcc does not exist yet. The ARC compiler supports -really-nostdlib to
override the default behavior.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>