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_span/src/symbol.rs

@@ -379,6 +379,7 @@ symbols! {
         abi_avr_interrupt,
         abi_c_cmse_nonsecure_call,
         abi_efiapi,
+        abi_gpu_kernel,
         abi_msp430_interrupt,
         abi_ptx,
         abi_riscv_interrupt,