Skip GPL-only symbols test when cross-compiling

This test depends on being able to execute the resulting binary
which will be impossible when cross-compiling.  Instead make a
worst case assumption which allows the build to continue as
recommended by the autoconf manual.

https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Runtime.html

Signed-off-by: Kamil Domański <kamil@domanski.co>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: tuxoko <tuxoko@gmail.com>
Closes openzfs/spl#507
Closes openzfs#4075

config/spl-build.m4

@@ -683,6 +683,7 @@ AC_DEFUN([SPL_AC_TEST_MODULE],
 		AC_DEFINE([SPL_IS_GPL_COMPATIBLE], [1],
 		    [Define to 1 if GPL-only symbols can be used])
 	], [
+	], [
 	])
 ])