[boot] Make sure the kernel entrypoint abi is sysv

Adding this now because I'm sure I'll forget later. Make sure to
annotate the entrypoint function pointer as `__attribute__((sysv_abi))`
so that it's not called via ms abi like the rest of the loader.

src/include/kernel_args.h

@@ -84,6 +84,6 @@ __attribute__((aligned(alignof(max_align_t))));
 
 } // namespace args
 
-using entrypoint = void (*)(args::header *);
+using entrypoint = __attribute__((sysv_abi)) void (*)(args::header *);
 
 } // namespace kernel