gallivm: improve dumping of bitcode

gallivm: improve dumping of bitcode

Use GALLIVM_DEBUG=dumpbc for dumping of modules as bitcode.
Instead of a fixed llvmpipe.bc name, use ir_<modulename>.bc so multiple
modules can be dumped (albeit it might still overwrite previous modules,
particularly the modules from draw tend to always have the same name).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>