[dev.cc] cmd/asm: final fixups for correct assembly of runtime, the last package...

[dev.cc] cmd/asm: final fixups for correct assembly of runtime, the last package to verify

- obj: add a missing setting of the context for a generated JMP instruction
- asm:  correct the encoding of mode (R)(R*scale)
- asm: fix a silly bug in the test for macro recursion.
- asm: accept address mode sym(R)(R*8); was an oversight

Change-Id: I27112eaaa1faa0d2ba97e414f0571b70733ea087
Reviewed-on: https://go-review.googlesource.com/4502
Reviewed-by: Russ Cox <rsc@golang.org>