nios2: Fix libgcc location detection

The extra CFLAGS required for correct operation of the toolchain are not
propagated into the libgcc detection code. In case of the compiler from
poky sdk, these extra CFLAGS contain the location of compiler sysroot
(the --sysroot= option) and without this option, the location of libgcc
is not properly reported. Add the missing CFLAGS to address this issue.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <lftan@altera.com>
Cc: Romain Perier <romain.perier@free-electrons.com>
Acked-by: Romain Perier <romain.perier@free-electrons.com>
Acked-by: Ley Foon Tan <lftan@altera.com>