[iOS][GN] Port ios_net_unittests to build with gn.

Add new BUILD.gn file exporting //ios/net and //ios/net:ios_net_unittests
targets (corresponding to ios_net and ios_net_unittests with gyp) and add
dependency on the unit test target in //BUILD.gn.

Add missing dependency on //build/config/mac:compiler when targetting iOS
or mac in //build/config/compiler/BUILD.gn (paralleling the other deps on
//build/config/${system}:compiler).

Change the "-arch" value to target fat architecture "arm64" & "armv7" when
current_cpu is "arm" (better to have fat binaries working on all devices
since it is not possible currently to target "armv7" with gn).

BUG=459705

Review URL: https://codereview.chromium.org/1388983003

Cr-Commit-Position: refs/heads/master@{#352827}

• BUILD.gn[diff]
• build/config/compiler/BUILD.gn[diff]
• build/config/mac/BUILD.gn[diff]
• ios/net/BUILD.gn[Added - diff]
• ios/net/ios_net.gyp[diff]
• ios/net/ios_net_unittests.gyp[diff]