gallium: uclib doesn't know backtrace_symbols

fix build in gcw0
driver: single buffer fallback

The gcw0 kernel currently makes only enough framebuffer memory
available for one 320x240 buffer.

Support variable number of buffers in bswap. This allows single
buffering, and (untested) triple-buffering as well.
Merge pull request #2 from laanwj/master

update to master