[llvm] [clang] [AArch64] Add soft-float ABI (PR #74460)

[via cfe-commits]

ktkachov-arm wrote:

Changing the ABI through the architecture flags i.e. +nofp has potential to greatly confuse users, who may come from other architectures that have soft-float ABIs already (including 32-bit arm)

- Does this also happen when using +nosimd?
- What about users like the Linux kernel that build with -mgeneral-regs-only?
- There is already an -mabi= option. How come it doesn't affect what is a pretty major ABI decision?

https://github.com/llvm/llvm-project/pull/74460