Based on the ppc64 port.

s390x supports 2, 4 and 6 byte instructions and Go assembly
instructions sometimes map to several s390x instructions. The
assembler loops until a fixed point is reached in order to use
branch instructions that can only handle a short offset in a
similar way to other ports.

Change-Id: I4278bf46aca35a96ca9cea0857e6229643c9c1e3
Reviewed-on: https://go-review.googlesource.com/20942Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>