Be more explicit on where the gcc dirs are in the bootstrap

It's possible for the gcc binary to be in /tools, but the libs we want
to be in /usr/lib. gcc will by default look in a dir related to where it
found the gcc binary.

buildstream/data/build-module.sh.in

diff --git a/buildstream/data/build-module.sh.in b/buildstream/data/build-module.sh.in
index d67ee9a..0f13706 100644
--- a/buildstream/data/build-module.sh.in
+++ b/buildstream/data/build-module.sh.in
@@ -26,10 +26,14 @@
 cp -a "$SRCDIR/{name}/." '{build_root}'
 cd '{build_root}'
 
+# dodgy hack because /usr/bin is a symlink to /tools/bin
+export GCC_EXEC_PREFIX='/usr/lib/gcc/'
+
 export PREFIX='{install_root}'
 
 export {variables}
 
+
 # Build the module
 {commands}