Detect calls to compiler intrinsics and emit an extern declarations

Detect calls to compiler intrinsics and emit an extern declarations
only for those. These extern declarations to intrinsics are currently
being emitted at the bottom of generated .s file, which works fine with
gpasm(not sure about MPSAM though).
PIC16 linker generates errors for few cases (function-args/struct_args_5) if you do not include any
extern declarations (even if no intrinsics are being used), but that
needs to be fixed in the linker itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71423 91177308-0d34-0410-b5e6-96231b3b80d8