when gcc is built --with-build-sysroot then $CC for the target libs
includes --sysroot, but when the test is run for these target libs
CC=xgcc is used without --sysroot

libstdc++ tests have special workaround to pass build time CC and
CFLAGS settings to the test system, same for libgfortran, but
libatomic or libgomp tests fail when --with-build-sysroot is used.

it was reported earlier but i could not find it in bugzilla:
https://gcc.gnu.org/ml/gcc-patches/2013-05/msg01181.html

the root cause is that the dejagnu 'find_gcc' function is used
as CC in the tests which is wrong if any special flag
(like --sysroot) is needed for compilation/linking.