Split out the low-level code-generation code to separate files.

Rev: src/Makefile.in:1.270
Rev: src/code/README.txt:1.1
Rev: src/code/bytecode.c:1.1
Rev: src/code/bytecode.h:1.1
Rev: src/code/computedgoto.c:1.1
Rev: src/code/computedgoto.h:1.1
Rev: src/code/ia32.c:1.1
Rev: src/code/ia32.h:1.1
Rev: src/code/sparc.c:1.1
Rev: src/code/sparc.h:1.1
Rev: src/peep.c:1.62
Rev: src/pikecode.c:1.1
Rev: src/pikecode.h:1.1