[dev.cc] cmd/asm: the assembler proper

[dev.cc] cmd/asm: the assembler proper

Add main.go, the simple driver for the assembler, and the
subdirectory internal/asm, which contains the parser and
instruction generator.

It's likely that much of the implementation is superstition,
or at best experimental phenomenology, but it does generate
working binaries.

Change-Id: I322a9ae8a20174b6693153f30e39217ba68f8032
Reviewed-on: https://go-review.googlesource.com/3196
Reviewed-by: Russ Cox <rsc@golang.org>