libgo: fix DejaGNU testsuite compiler when using build sysroot

Patch from Thomas Schwinge.

PR testsuite/109951
* configure.ac: 'AC_SUBST(SYSROOT_CFLAGS_FOR_TARGET)'.
* Makefile.in: Regenerate.
* configure: Likewise.
* testsuite/Makefile.in: Likewise.
* testsuite/lib/libgo.exp (libgo_init): If
'--with-build-sysroot=[...]' was specified, use it for build-tree
testing.
* testsuite/libgo-test-support.exp.in (GOC_UNDER_TEST): Don't set.
(SYSROOT_CFLAGS_FOR_TARGET): Set.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/527755