cmd/gc: move reg.c into portable code

cmd/gc: move reg.c into portable code

Now there is only one registerizer shared among all the systems.
There are some unfortunate special cases based on arch.thechar
in reg.c, to preserve bit-for-bit compatibility during the refactoring.
Most are probably bugs one way or another and should be revisited.

Change-Id: I153b435c0eaa05bbbeaf8876822eeb6dedaae3cf
Reviewed-on: https://go-review.googlesource.com/3883
Reviewed-by: Austin Clements <austin@google.com>