configure: Force ucontext check to fail on ARM

Force the check for ucontext to fail on ARM hosts, because they
implement it to return ENOSYS all the time.

TODO: this is a temporary commit to get things running; I'm not
sure exactly what the right solution here is but this isn't it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/configure b/configure
index c21d5eb..5074a63 100755
--- a/configure
+++ b/configure
@@ -2586,7 +2586,7 @@
 # check if we have makecontext
 
 ucontext_coroutine=no
-if test "$darwin" != "yes"; then
+if test "$darwin" != "yes" && test "$cpu" != "arm"; then
   cat > $TMPC << EOF
 #include <ucontext.h>
 int main(void) { makecontext(0, 0, 0); }