Auto merge of #52032 - DiamondLovesYou:amdgpu-kernel-abi, r=alexcrichton

Add the `amdgpu-kernel` ABI.

Technically, there are requirements imposed by the LLVM
`AMDGPUTargetMachine` on functions with this ABI (eg, the return type
must be void), but I'm unsure exactly where this should be enforced.

src/librustc_codegen_llvm/abi.rs

@@ -343,6 +343,7 @@ impl<'a, 'tcx> FnTypeExt<'a, 'tcx> for FnType<'tcx, Ty<'tcx>> {
             PtxKernel => Conv::PtxKernel,
             Msp430Interrupt => Conv::Msp430Intr,
             X86Interrupt => Conv::X86Intr,
+            AmdGpuKernel => Conv::AmdGpuKernel,
 
             // These API constants ought to be more specific...
             Cdecl => Conv::C,
@@ -608,6 +609,7 @@ impl<'a, 'tcx> FnTypeExt<'a, 'tcx> for FnType<'tcx, Ty<'tcx>> {
     fn llvm_cconv(&self) -> llvm::CallConv {
         match self.conv {
             Conv::C => llvm::CCallConv,
+            Conv::AmdGpuKernel => llvm::AmdGpuKernel,
             Conv::ArmAapcs => llvm::ArmAapcsCallConv,
             Conv::Msp430Intr => llvm::Msp430Intr,
             Conv::PtxKernel => llvm::PtxKernel,