Compilation for Intel MCU (Part 2/3)

This is the second patch required to support compilation for Intel MCU target (e.g. Intel(R) Quark(TM) micro controller D 2000).
When IAMCU triple is used:
 * Recognize and use IAMCU GCC toolchain
 * Set up include paths
 * Forbid C++

Differential Revision: http://reviews.llvm.org/D19274

llvm-svn: 272883