[Darwin-arm/iphone] Add support for clang on iOS.

(cherry picked from commit 5f96e634fdd4909db3792c873f6a4179f0897f56)

From: CoolStar <coolstarorganization@gmail.com>

git-svn-id: http://svn.howett.net/svn/theos/trunk@635 4410221e-0ddf-4ce3-99c0-2db6c0dbc727
[Darwin-arm/iphone] Don't break if the user requests iphone:gcc.

From: Dustin L. Howett <dustin@howett.net>

git-svn-id: http://svn.howett.net/svn/theos/trunk@636 4410221e-0ddf-4ce3-99c0-2db6c0dbc727
[deb] Do not break if the user has layout/ but no layout/DEBIAN.

From: Dustin L. Howett <dustin@howett.net>

git-svn-id: http://svn.howett.net/svn/theos/trunk@637 4410221e-0ddf-4ce3-99c0-2db6c0dbc727