[Driver] Error if ARM mode was selected explicitly for M-profile CPUs.

Summary:
M-class profiles do not support ARM execution mode, so providing
-marm/-mno-thumb does not make sense in combination with -mcpu/-march
options that support the M-profile.

This is a follow-up patch to D35569 and it seemed pretty clear that we
should emit an error in the driver in this case.

We probably also should warn/error if the provided -mcpu/-march options
do not match, e.g. -mcpu=cortex-m0 -march=armv8-a is invalid, as
cortex-m0 does not support armv8-a. But that should be a separate patch
I think.


Reviewers: echristo, richard.barton.arm, rengolin, labrinea, charles.baylis

Reviewed By: rengolin

Subscribers: aemerson, javed.absar, kristof.beyls, cfe-commits

Differential Revision: https://reviews.llvm.org/D35826

llvm-svn: 310047