amdgcn: Replace asm("s8") by __builtin_gcn_kernarg_ptr if existing

Check whether __builtin_gcn_kernarg_ptr is available and, if it is,
call it instead using the hard-coded 'asm("s8")' in:
* newlib/libc/machine/amdgcn/exit-value.h (exit_with_int)
* newlib/libc/machine/amdgcn/mlock.c (sbrk)
* newlib/libc/sys/amdgcn/write.c (write)

 newlib/libc/machine/amdgcn/exit-value.h |  6 ++++++
 newlib/libc/machine/amdgcn/mlock.c      | 10 +++++++---
 newlib/libc/sys/amdgcn/write.c          |  4 ++++
 3 files changed, 17 insertions(+), 3 deletions(-)

newlib/libc/machine/amdgcn/exit-value.h

@@ -21,8 +21,14 @@ exit_with_int (int val)
 {
   /* Write the exit value to the conventional place.  */
   int *return_value;
+#if defined(__has_builtin) && __has_builtin(__builtin_gcn_kernarg_ptr)
+  asm ("s_load_dwordx2	%0, %1, 16 glc\n\t"
+       "s_waitcnt	0"
+       : "=Sg"(return_value) : "r"(__builtin_gcn_kernarg_ptr()));
+#else
   asm ("s_load_dwordx2	%0, s[8:9], 16 glc\n\t"
        "s_waitcnt	0" : "=Sg"(return_value));
+#endif
   *return_value = val;
 
   /* Terminate the current kernel.  */

newlib/libc/machine/amdgcn/mlock.c

@@ -39,11 +39,15 @@ sbrk (ptrdiff_t nbytes)
 {
   if (__heap_ptr == (char *)-1)
     {
-      /* Find the heap from kernargs.
+      /* Find the heap from kernargs.  */
-         The kernargs pointer is in s[8:9].
-	 This will break if the enable_sgpr_* flags are ever changed.  */
       char *kernargs;
+#if defined(__has_builtin) && __has_builtin(__builtin_gcn_kernarg_ptr)
+      kernargs = __builtin_gcn_kernarg_ptr ();
+#else
+      /* The kernargs pointer is in s[8:9].
+	 This will break if the enable_sgpr_* flags are ever changed.  */
       asm ("s_mov_b64 %0, s[8:9]" : "=Sg"(kernargs));
+#endif
 
       /* The heap data is at kernargs[3].  */
       struct heap *heap = *(struct heap **)(kernargs + 24);

newlib/libc/sys/amdgcn/write.c

@@ -56,7 +56,11 @@ _READ_WRITE_RETURN_TYPE write (int fd, const void *buf, size_t count)
     }
 
   /* The output data is at ((void*)kernargs)[2].  */
+#if defined(__has_builtin) && __has_builtin(__builtin_gcn_kernarg_ptr)
+  register void **kernargs = __builtin_gcn_kernarg_ptr ();
+#else
   register void **kernargs asm("s8");
+#endif
   struct output *data = (struct output *)kernargs[2];
 
   /* Each output slot allows 256 bytes, so reserve as many as we need. */