Reinstate aapcs as the default arm abi.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224490 91177308-0d34-0410-b5e6-96231b3b80d8
Make sure that arm-linux-gnu is still the apcs-gnu ABI when we
use clang -cc1 matching the front end and backend. Fix up a couple
of tests that were testing aapcs for arm-linux-gnu.

The test that removes the aapcs abi calling convention removes
them because the default triple matches what the backend uses
for the calling convention there and so it doesn't need to be
explicitly stated - see the code in TargetInfo.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224491 91177308-0d34-0410-b5e6-96231b3b80d8