Model ARM backend ABI selection after the front end code doing the
same. This will change the "bare metal" ABI from APCS to AAPCS.

The only difference between the front and back end code is that
the code for Triple::GNU was added for environment. That will migrate
to the front end shortly.

Tests updated with the ABI they were originally testing in the case
of bare metal (e.g. -mtriple armv7) or with a -gnu for arm-linux
triples.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224489 91177308-0d34-0410-b5e6-96231b3b80d8
Add a new string member to the TargetOptions struct for the name
of the abi we should be using. For targets that don't use the
option there's no change, otherwise this allows external users
to set the ABI via string and avoid some of the -backend-option
pain in clang.

Use this option to move the ABI for the ARM port from the
Subtarget to the TargetMachine and update the testcases
accordingly since it's no longer valid to set via -mattr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224492 91177308-0d34-0410-b5e6-96231b3b80d8