toolchain: arcmwdt: don't set STDC_LIB_EXT1 macro in case of minimal libc

Don't provide __STDC_LIB_EXT1__ macro (Extensions to the C Library,
Part 1: Bounds-checking interfaces) if we use minimal libc - as we don't
have functions from this extension implemented in minimal libc.

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