[SystemZ][z/OS] Initial implementation for lowerCall on z/OS

- This patch provides the initial implementation for lowering a call on z/OS according to the XPLINK64 calling convention
- A series of changes have been made to SystemZCallingConv.td to account for these additional XPLINK64 changes including adding a new helper function to shadow the stack along with allocation of a register wherever appropriate
- For the cases of copying a f64 to a gr64 and a f128 / 128-bit vector type to a gr64, a `CCBitConvertToType` has been added and has been bitcasted appropriately in the lowering phase
- Support for the ADA register (R5) will be provided in a later patch.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D111662

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 08295df..390457d 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -135,6 +135,9 @@ bool TargetMachine::shouldAssumeDSOLocal(const Module &M,
     return true;
   }
 
+  if (TT.isOSBinFormatGOFF())
+    return true;
+
   if (TT.isOSBinFormatMachO()) {
     if (RM == Reloc::Static)
       return true;