system/trrntzip: ARCH fixes and cleanup.

Signed-off-by: dsomero <xgizzmo@slackbuilds.org>

system/trrntzip/trrntzip.SlackBuild

@@ -22,13 +22,20 @@
 # OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 # ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-set -eu
 
 PRGNAM=trrntzip
 VERSION=v02
-ARCH=${ARCH:-i486}
 BUILD=${BUILD:-1}
 TAG=${TAG:-_SBo}
+
+if [ -z "$ARCH" ]; then
+  case "$( uname -m )" in
+    i?86) ARCH=i486 ;;
+    arm*) ARCH=arm ;;
+       *) ARCH=$( uname -m ) ;;
+  esac
+fi
+
 CWD=`pwd`
 TMP=${TMP:-/tmp/SBo}
 PKG=$TMP/package-$PRGNAM
@@ -42,6 +49,8 @@ elif [ "$ARCH" = "x86_64" ]; then
   SLKCFLAGS="-O2 -fPIC"
 fi
 
+set -eu
+
 rm -rf $PKG
 mkdir -p $TMP $PKG
 cd $TMP