[ARM] Restructure cpu handling in the driver to mostly use the triple

Using the target cpu to determine some behaviour is sprinkled in
several places in the driver, but in almost all the information that
is needed can be found in the triple. Restructure things so that the
triple is used, and the cpu is only used if the exact cpu name is
needed.

Also add a check that the -mcpu argument is valid, and correct the
-march argument checking so that it handles -march=native correctly. I
would have liked to move these checks into the computation of the
triple, but the triple is calculated several times in several places
and that would lead to multiple error messages for the same thing.

Differential Revision: http://reviews.llvm.org/D9879

llvm-svn: 237894