[dev.ssa] cmd/compile: handle Div, Convert, GetClosurePtr etc. on ARM

[dev.ssa] cmd/compile: handle Div, Convert, GetClosurePtr etc. on ARM

This CL adds support of Div, Mod, Convert, GetClosurePtr and 64-bit indexing
support to SSA backend for ARM.

Add tests for 64-bit indexing to cmd/compile/internal/gc/testdata/string_ssa.go.

Tests cmd/compile/internal/gc/testdata/*_ssa.go passed, except compound_ssa.go
and fp_ssa.go.

Progress on SSA for ARM. Still not complete. Essentially the only unsupported
part is floating point.

Updates #15365.

Change-Id: I269e88b67f641c25e7a813d910c96d356d236bff
Reviewed-on: https://go-review.googlesource.com/23542
Reviewed-by: David Chase <drchase@google.com>