selftests: vDSO - fix to exclude x86 test on non-x86 platforms

Fix to exclude vdso_standalone_test_x86 test from building on non-x86
platforms. In addition, fix it to use TEST_GEN_PROGS which is the right
variable to use for generated programs. TEST_PROGS is for shell scripts.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>