[clang] [clang][AArch64] Enable fp128 for aarch64 linux target (PR #85070)

[via cfe-commits]

lntue wrote:

> Right, I'm more interested in being able to use on aarch64 the `_Float128` type as standardized by C23, than the clang specific `__float128` extension. Does this PR help get us there?
> 
> EDIT: in fact, if we could skip propagating the clang-specific extension further, and just go straight to C23 `_Float128`, I'd prefer that.

I would second that we should go straight to C23 `_Float128`.  The current landscape for float128 supports in C (clang / gcc) and C++ (clang++, g++) : https://github.com/llvm/llvm-project/pull/78017#discussion_r1476974657
So adding `__float128` to aarch64 does not help much to simplify codes that need to work for both clang & gcc.

I haven't tested but I think the picture for float128 on riscv would be similar to aarch64.

https://github.com/llvm/llvm-project/pull/85070