[Clang FE] Recognize -mnop-mcount CL option (SystemZ only).

Recognize -mnop-mcount from the command line and add a function attribute
"mnop-mcount"="true" when passed.

When this option is used, a nop is added instead of a call to fentry. This
is used when building the Linux Kernel.

If this option is passed for any other target than SystemZ, an error is
generated.

Review: Ulrich Weigand
https://reviews.llvm.org/D67763