Various assemble()-related cleanups:

  Moved the global variable store_linenumbers to an argument of assemble().
  assemble() and docode() now return the entrypoint of the generated code.
  Support for ALIGN_PIKE_FUNCTION_BEGINNINGS has moved to assemble().
  Some lowlevel opcode stuff has been moved from eval_low() to docode().
  Moved call of ADD_COMPILED() from the individual files in code/ to assemble().
  Added pseudo opcodes F_FILENAME and F_LINE for the benefit of PIKE_PORTABLE_BYTECODE.
  assemble() now has some support for PIKE_PORTABLE_BYTECODE.

Rev: src/code/bytecode.c:1.7
Rev: src/code/computedgoto.c:1.4
Rev: src/code/ia32.c:1.36
Rev: src/code/ppc32.c:1.33
Rev: src/code/sparc.c:1.38
Rev: src/docode.c:1.173
Rev: src/docode.h:1.19
Rev: src/las.c:1.343
Rev: src/opcodes.c:1.161
Rev: src/opcodes.h:1.38
Rev: src/peep.c:1.93
Rev: src/peep.h:1.14