Correctly identify iOS simulator processes in debugserver.

Starting with iOS 13 simulator binaries are identified with an
explicit platform in the new LC_BUILD_VERSION load command.

On older deployment targets using the LC_VERSION_MIN load commands,
this patch detects when an ios process runs on a macOS host and
updates the target triple with the "simulator" environment
accordingly.

(Patch re-applied with bugfix this time).

This is part of https://bugs.swift.org/browse/SR-11971

rdar://problem/58438125

Differential Revision: https://reviews.llvm.org/D75696