Since commit 0cce556b (Xcode: Use sysroot and deployment target to
identify compiler, 2014-04-29) our compiler id detection project uses
the target platform SDK in case Xcode selects a different compiler based
on it.  Now the compiler id project actually compiles with the target
compiler and SDK when cross-compiling.

The iOS tools do not support the 'com.apple.product-type.tool' product
type we use in our compiler id detection project.  When targeting
iPhone, use product type 'com.apple.product-type.bundle.unit-test'
instead.