[NVPTX] Fix DwarfFrameBase construction (llvm#101000)

The `{0}` here was initializing the first union member `Register`,
rather than the union member used by CFA, which is `Offset`. Prior to
llvm#99263 this was harmless, but
now they have different layout, leading to test failures on some
platforms (at least i686 and s390x).

(cherry picked from commit 842a332)

Author: nikic

llvm/lib/Target/NVPTX/NVPTXFrameLowering.cpp

@@ -93,5 +93,8 @@ MachineBasicBlock::iterator NVPTXFrameLowering::eliminateCallFramePseudoInstr(
 
 TargetFrameLowering::DwarfFrameBase
 NVPTXFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
-  return {DwarfFrameBase::CFA, {0}};
+  DwarfFrameBase FrameBase;
+  FrameBase.Kind = DwarfFrameBase::CFA;
+  FrameBase.Location.Offset = 0;
+  return FrameBase;
 }