ARC: MWDT: generate access to builtin funcs only if ARCMWDT_LIBC is used

The MWDT compiler can replace some code with call to builtin
functions. We can't rely on these functions presence if we
don't use MWDT libc.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>