compiler: internally merge Conv::PtxKernel into GpuKernel

It is speculated that these two can be conceptually merged, and it can
start by ripping out rustc's notion of the PtxKernel call convention.
Leave the ExternAbi for now, but the nvptx target now should see it as
just a different way to spell Conv::GpuKernel.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -687,7 +687,6 @@ impl llvm::CallConv {
             Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,
             Conv::ArmAapcs => llvm::ArmAapcsCallConv,
             Conv::Msp430Intr => llvm::Msp430Intr,
-            Conv::PtxKernel => llvm::PtxKernel,
             Conv::X86Fastcall => llvm::X86FastcallCallConv,
             Conv::X86Intr => llvm::X86_Intr,
             Conv::X86Stdcall => llvm::X86StdcallCallConv,