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/stable_mir/src/abi.rs

@@ -442,6 +442,8 @@ pub enum CallConvention {
 
     PtxKernel,
 
+    GpuKernel,
+
     X86Fastcall,
     X86Intr,
     X86Stdcall,

compiler/stable_mir/src/ty.rs

@@ -1077,6 +1077,7 @@ pub enum Abi {
     PtxKernel,
     Msp430Interrupt,
     X86Interrupt,
+    GpuKernel,
     EfiApi,
     AvrInterrupt,
     AvrNonBlockingInterrupt,