perf test coresight: Dismiss clang warning for unroll loop thread

clang-18.1.3 on Ubuntu 24.04.2 reports warning:

unroll_loop_thread.c:35:25: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
35 | : /* in */ [in] "r" (in)
| ^
unroll_loop_thread.c:39:1: warning: non-void function does not return a value [-Wreturn-type]
39 | }
| ^

Use the modifier "w" for 32-bit register access and return NULL at the
end of thread function.

Signed-off-by: Leo Yan <[e-mail]>
Reviewed-by: Ian Rogers <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: Palmer Dabbelt <[e-mail]>
Cc: Albert Ou <[e-mail]>
Cc: Alexandre Ghiti <[e-mail]>
Cc: Nick Desaulniers <[e-mail]>
Cc: Justin Stitt <[e-mail]>
Cc: Bill Wendling <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Nathan Chancellor <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Paul Walmsley <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/shell/coresight/unroll_loop_thread/unroll_loop_thread.c

···
 	for (i = 0; i < 10000; i++) {
 		asm volatile (
 // force an unroll of thia add instruction so we can test long runs of code
-#define SNIP1 "add %[in], %[in], #1\n"
+#define SNIP1 "add %w[in], %w[in], #1\n"
 // 10
 #define SNIP2 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1 SNIP1
 // 100
···
 			: /* clobber */
 		);
 	}
+
+	return NULL;
 }
 
 static pthread_t new_thr(void *(*fn) (void *arg), void *arg)