[dev.cc] cmd/asm: delete overflow checking, make labels function-scoped

[dev.cc] cmd/asm: delete overflow checking, make labels function-scoped

The overflow checking was causing more problems than it was avoiding,
so get rid of it. But because arithmetic is done with uint64s, to simplify
dealing with large constants, complain about right shift and divide with
huge numbers to avoid ambiguity about signed shifts.

Change-Id: I5b5ea55d8e8c02846605f4a3f8fd7a176b1e962b
Reviewed-on: https://go-review.googlesource.com/3531
Reviewed-by: Russ Cox <rsc@golang.org>