Configure: move --noexecstack probe to Configure.

config probe doesn't work in cross-compile scenarios or with clang.
In addition consolidate -Qunused-arguments handling.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6174)
diff --git a/Configurations/15-android.conf b/Configurations/15-android.conf
index 9df4d94..ddd642a 100644
--- a/Configurations/15-android.conf
+++ b/Configurations/15-android.conf
@@ -65,7 +65,7 @@
             my ($api, $arch) = ($1, $2);
 
             my $triarch = $triplet{$arch};
-            my $cflags = "-Wa,--noexecstack";
+            my $cflags;
             my $cppflags;
 
             # see if there is NDK clang on $PATH