gallivm: use sstream for dissasembling

gallivm: use sstream for dissasembling

Currently, disassemble() directly prints to stdout. This has broke the
profiling support for llvmpipe JIT code.

This patch redirects the output to an sstream object, which is then
either gets printed to stdout (for assembly debugging) or gets written
to a file in /tmp/ (for profiling support).

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>