[test-android] fix gettext build in build.sh by distinct --host and --target.

gettext is known as cause of problem for running host tool in target ABI
in our builds, and the latest script still had the issue.
https://github.com/atsushieno/aap-lv2-fluidsynth/actions/runs/914453645

This fixes the problem by explicitly specifying that the build host is
x86_64 linux.
If you want to reuse the script on other platforms, you have to edit it.

test-android/build-scripts/build.sh

@@ -82,7 +82,8 @@
         set -ex
         pushd $DEV/gettext-$GETTEXT_VERSION
         ./configure \
-          --host=$AUTOTOOLS_TARGET \
+          --host=x86_64-pc-linux \
+          --target=$AUTOTOOLS_TARGET \
           --prefix=$PREFIX \
           --libdir=$LIBPATH0 \
           --disable-rpath \