Do not build testglue if it is not required

ARC newlib doesn't have function _exit, however Dejagnu testglue is intended
to wrap around it. Thus function _exit() gets created even though it is
never called. When performing execution tests Dejagnu sets breakpoints at
"exit" functions: abort() and _exit(), if it couldn't set at _exit() then at
exit(). That latter case should be applied to ARC, but instead we get
breakpoint at function created by testglue which is never called.

This patch adds a conditional clause that checks whether user explicitly
requested to not use testglue (aka status wrapper). This is the case with
ARC: we don't need testglue, so we disable it. GCC testsuite already has
such condition clauses using `board_info needs_status_wrapper` and this
patch uses the very same variable.