Use the 64-bit linker for target mips64r6 in buildbot-build.sh.

Change-Id: I51fddf0aecb2c834d42426ce22347d4a2dbed3d8
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index 1dced32..2ee87e5 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -27,9 +27,9 @@
 showcommands=
 make_command=
 
-if [[ "$TARGET_PRODUCT" == "armv8" ]]; then
-  linker="linker64"
-fi
+case "$TARGET_PRODUCT" in
+  (armv8|mips64r6) linker="linker64";;
+esac
 
 if [[ "$ART_TEST_ANDROID_ROOT" != "" ]]; then
   android_root="$ART_TEST_ANDROID_ROOT"