Add gpu-kernel calling convention

The amdgpu-kernel calling convention was reverted in commit
f6b21e90d1 due to inactivity in the amdgpu
target.

Introduce a `gpu-kernel` calling convention that translates to
`ptx_kernel` or `amdgpu_kernel`, depending on the target that rust
compiles for.

compiler/rustc_target/src/json.rs

@@ -113,6 +113,7 @@ impl ToJson for crate::abi::call::Conv {
             Self::X86VectorCall => "X86VectorCall",
             Self::X86_64SysV => "X86_64SysV",
             Self::X86_64Win64 => "X86_64Win64",
+            Self::GpuKernel => "GpuKernel",
             Self::AvrInterrupt => "AvrInterrupt",
             Self::AvrNonBlockingInterrupt => "AvrNonBlockingInterrupt",
             Self::RiscvInterrupt { kind } => {