AppImage: Improve binary stripping

Slightly reduces file size, improves build speed and makes build more
reproducible.

The .comment section contained GCC version information which could cause
different build output from just a minor update in GCC. The information is not
needed so we strip this.

The strip command was invoked using xargs, spawning a new process for each file.
This is inefficient as xargs can correctly run the strip command with multiple
file names.

-----

taken from 43aaf9572f

contrib/build-linux/appimage/build.sh

@@ -178,13 +178,14 @@ info "Copying additional libraries"
 
 info "stripping binaries from debug symbols."
 # "-R .note.gnu.build-id" also strips the build id
+# "-R .comment" also strips the GCC version information
 strip_binaries()
 {
   chmod u+w -R "$APPDIR"
   {
     printf '%s\0' "$APPDIR/usr/bin/python3.6"
     find "$APPDIR" -type f -regex '.*\.so\(\.[0-9.]+\)?$' -print0
-  } | xargs -0 --no-run-if-empty --verbose -n1 strip -R .note.gnu.build-id
+  } | xargs -0 --no-run-if-empty --verbose strip -R .note.gnu.build-id -R .comment
 }
 strip_binaries