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/spec/mod.rs

@@ -2854,6 +2854,7 @@ impl Target {
             }
             Win64 { .. } | SysV64 { .. } => self.arch == "x86_64",
             PtxKernel => self.arch == "nvptx64",
+            GpuKernel => ["amdgpu", "nvptx64"].contains(&&self.arch[..]),
             Msp430Interrupt => self.arch == "msp430",
             RiscvInterruptM | RiscvInterruptS => ["riscv32", "riscv64"].contains(&&self.arch[..]),
             AvrInterrupt | AvrNonBlockingInterrupt => self.arch == "avr",