Compilation for Intel MCU (Part 3/3)

This is the last patch required to support compilation for Intel MCU target (e.g. Intel(R) Quark(TM) micro controller D 2000).

When IAMCU triple is used:
 * Use IAMCU linker output format
 * Link with IAMCU crt objects
 * Link with IAMCU libraries

Differential Revision: http://reviews.llvm.org/D20675

llvm-svn: 272885