Changes to CodeGen_C to allow result to compile & run properly

-- provide only a single definition of buffer_t, rather than two
different ones (ahem)
-- Added additional wrapper functions for a few more externs (ceil_f32,
floor_f32, etc). Note: I didn't attempt to exhaustively provide all of
them, so more boring work might be needed here
-- Added Intrinsic handling for the bitwise ops and reinterpret
-- Added void-casting to work around "unused variable" warnings in a
few cases

(There's some noise in the diff due to stripping of trailing
whitespace; if this is unacceptable then I'll resubmit)