Add gpu-kernel calling convention

The amdgpu-kernel calling convention was reverted in commit
f6b21e9 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.

Author: Flakebi

src/abi/mod.rs

@@ -65,7 +65,11 @@ pub(crate) fn conv_to_call_conv(sess: &Session, c: Conv, default_call_conv: Call
             sess.dcx().fatal("C-cmse-nonsecure-entry call conv is not yet implemented");
         }
 
-        Conv::Msp430Intr | Conv::PtxKernel | Conv::AvrInterrupt | Conv::AvrNonBlockingInterrupt => {
+        Conv::Msp430Intr
+        | Conv::PtxKernel
+        | Conv::GpuKernel
+        | Conv::AvrInterrupt
+        | Conv::AvrNonBlockingInterrupt => {
             unreachable!("tried to use {c:?} call conv which only exists on an unsupported target");
         }
     }