[clang][ARM] follow GCC behavior for defining __SOFTFP__

GCC behavior regarding defining __SOFTFP__ when (implicitly) specifying
-mfloat-abi=softfp:
- compile without (implicit) FP: define __SOFTFP__
- compile with (implicit) FP: don't define __SOFTFP__

Currently Clang doesn't define __SOFTFP__ when softfp is specified, either with
or without FP. This patch brings Clang in line with GCC behavior.

This was raised by itaig1 over on Github:
https://github.com/llvm/llvm-project/issues/55755

Reviewed By: pratlucas

Differential Revision: https://reviews.llvm.org/D135680