[AArch64] Fix PR34625 -mtune without -mcpu should not set -target-cpu

When -mtune is used on AArch64 the -target-cpu is passed the value of the
cpu given to -mtune. As well as setting micro-architectural features of the
-mtune cpu, this will also add the architectural features such as support
for instructions. This can result in the backend using instructions that
are supported in the -mtune cpu but not supported in the target
architecture. For example use of the v8.1-a LSE extensions with -march=v8.
    
This change removes the setting of -target-cpu for -mtune, the -mcpu must
be used to set -target-cpu. This has the effect of removing all non-hard
coded benefits of mtune but it does produce correct output when -mtune cpu
with a later architecture than v8 is used.
    
Fixes PR34625
    
Differential Revision: https://reviews.llvm.org/D39179




git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316424 91177308-0d34-0410-b5e6-96231b3b80d8