Support environment variables

The way we work around SR-1636 is we add a C language target with atbuild's new C project syntax.

This C target can access `environ` and the result follows.

I think this bootstraps OK, we'll see what CI says

See also: https://bugs.swift.org/browse/SR-1636, https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019350.html
2 jobs for master in 20 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
failed #6806
autoscale-linux
linux

00:00:20

failed #6807
openswift xcodeselect8
osx

00:00:05

 
Name Stage Failure
failed
linux Build There is an unknown failure, please try again
<unknown>:0: error: underlying Objective-C module 'atenvironment' not found
atenvironment/environment.swift:23:13: error: use of unresolved identifier '_environment'
let e = _environment()!
^~~~~~~~~~~~
atenvironment/environment.swift:21:12: note: did you mean 'environment'?
public var environment: [String: String] {
^
<unknown>:0: error: build had 1 command failures
ERROR: Build failed: exit code 1
failed
osx Build There is an unknown failure, please try again
<unknown>:0: error: underlying Objective-C module 'atenvironment' not found
atenvironment/environment.swift:23:13: error: use of unresolved identifier '_environment'
let e = _environment()!
^~~~~~~~~~~~
atenvironment/environment.swift:21:12: note: did you mean 'environment'?
public var environment: [String: String] {
^
<unknown>:0: error: build had 1 command failures
ERROR: Build failed: exit status 1