[Bug testsuite/71931] build sysroot flags are not passed to target lib tests

[nsz at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71931

--- Comment #1 from nsz at gcc dot gnu.org ---
a workaround is passing EXTRA_DEJAGNU_SITE_CONFIG=foo.exp to make, where
foo.exp has

set GCC_UNDER_TEST "build-dir/gcc/xgcc -Bbuild-dir/gcc --sysroot build-sysroot"