MSP430: Move __mspabi_mpyll calling conv config to tablegen (#153988)

There are several libcall choices for MUL_I64 which depend on the
subtarget, but this is the base case. The manual custom ISelLowering
is still overriding the decision until we have a way to control
lowering choices, but we can still get the calling convention
set for now.

llvm/include/llvm/IR/RuntimeLibcalls.td

@@ -2094,6 +2094,9 @@ def MSP430SystemLibrary
       __mspabi_remul,
       LibcallsWithCC<(add __mspabi_remull), MSP430_BUILTIN>,
 
+      // May not be used depending on subtarget
+      LibcallsWithCC<(add __mspabi_mpyll), MSP430_BUILTIN>,
+
       // Bitwise Operations - EABI Table 10
       // TODO: __mspabi_[srli/srai/slli] ARE implemented in libgcc
       __mspabi_srll,

llvm/lib/IR/RuntimeLibcalls.cpp

@@ -54,11 +54,6 @@ void RuntimeLibcallsInfo::initLibcalls(const Triple &TT,
 
     return;
   }
-
-  if (TT.getArch() == Triple::ArchType::msp430) {
-    setLibcallImplCallingConv(RTLIB::__mspabi_mpyll,
-                              CallingConv::MSP430_BUILTIN);
-  }
 }
 
 LLVM_ATTRIBUTE_ALWAYS_INLINE