[llvm] ARM: Remove unnecessary manual ABI lowering for sincos_stret (PR #166040)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue approved this pull request.

LGTM, though I think it'd be work regenerating the `llvm/test/CodeGen/ARM/sincos.ll` test case (it looks pretty weak as it only matches `bl ___sincosf_stret`). 

https://github.com/llvm/llvm-project/pull/166040