[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.
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 \