all: add GOOS=ios GOARCH=amd64 target for the ios simulator

The Go toolchain has supported the simulator for years, but always in
buildmode=c-archive which is intrinsically externally linked and PIE.

This CL moves that support from GOOS=darwin GOARCH=amd64 -tags=ios to
just GOOS=ios GOARCH=amd64 to match the change for iOS devices.

This change also forces external linking and defaults to buildmode=pie
to support Go binaries in the default buildmode to run on the simulator.

CL 255257 added the necessary support to the exec wrapper.

Updates #38485
Fixes #42100

Change-Id: I6e6ee0e8d421be53b31e3d403880e5b9b880d031
Reviewed-on: https://go-review.googlesource.com/c/go/+/263798


Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Elias Naur <mail@eliasnaur.com>