ARM: Stop setting sincos_stret calling convention (#147457)

This was going out of its way to explicitly mark these as
ARM_AAPCS_VFP. This has been explicitly set since 8b40366,
where the commit message states that "sincos" (not sincos_stret)
has a special calling convention. However, that commit also sets
the calling convention for all libcalls to ARM_AAPCS_VFP, and
getEffectiveCallingConv returns the same for CCC anyway in tests
using isWatchABI triples.

The net result of this appears to be a change in behavior when
using -float-abi=soft with isWatchABI, which have no tests so
I assume this is a theoretical combination.

If I assert
```
  if (getTargetMachine().getTargetTriple().isWatchABI()) {
    assert(!useSoftFloat());
    assert(getEffectiveCallingConv(CallingConv::C, false) == CallingConv::ARM_AAPCS_VFP);
  }
```
Only 2 tests fail the second condition, which look like copy paste
accidents
using v7k triples with linux and only needed a filler triple. This is a
consequence
of strangely using the target architecture in place of the OS ABI check,
as was done in 042a6c1

Author: arsenm

llvm/lib/IR/RuntimeLibcalls.cpp

@@ -169,12 +169,6 @@ void RuntimeLibcallsInfo::initLibcalls(const Triple &TT,
     if (darwinHasSinCosStret(TT)) {
       setLibcallImpl(RTLIB::SINCOS_STRET_F32, RTLIB::__sincosf_stret);
       setLibcallImpl(RTLIB::SINCOS_STRET_F64, RTLIB::__sincos_stret);
-      if (TT.isWatchABI()) {
-        setLibcallImplCallingConv(RTLIB::__sincosf_stret,
-                                  CallingConv::ARM_AAPCS_VFP);
-        setLibcallImplCallingConv(RTLIB::__sincos_stret,
-                                  CallingConv::ARM_AAPCS_VFP);
-      }
     }
 
     if (darwinHasExp10(TT)) {