Make sure a out/build_number.txt file is present with linux-bionic

On some branches if out/build_number.txt must be present for
linux_bionic targets to be built. This makes the build_linux_bionic.sh
script create this file.

Test: DIST_DIR=$PWD/fast-out/dist ./art/tools/dist_linux_bionic.sh showcommands com.android.runtime.host com.android.support.apexer
Bug: 132822173
Change-Id: Idc2e89e3947468f26d61d2db224910f3c13410d7
diff --git a/tools/build_linux_bionic.sh b/tools/build_linux_bionic.sh
index 0a933d4..50dc7d2 100755
--- a/tools/build_linux_bionic.sh
+++ b/tools/build_linux_bionic.sh
@@ -57,6 +57,7 @@
 # There is no good way to force soong to generate host-bionic builds currently
 # so this is a hacky workaround.
 tmp_soong_var=$(mktemp --tmpdir soong.variables.bak.XXXXXX)
+tmp_build_number=$(cat ${out_dir}/build_number.txt)
 
 cat $out_dir/soong/soong.variables > ${tmp_soong_var}
 
@@ -88,4 +89,7 @@
 
 rm $tmp_soong_var
 
+# Write a new build-number
+echo ${tmp_build_number}_SOONG_ONLY_BUILD > ${out_dir}/build_number.txt
+
 build/soong/soong_ui.bash --make-mode --skip-make $@