Add missing includes.

These missing includes were causing build warnings, but also a real bug in
which the "size" parameter to "write" was being passed in 32-bit, whereas it
ought to be 64-bit.  This led to intermittent bad behaviour.

newlib/libc/machine/amdgcn/abort.c

@@ -13,7 +13,7 @@
  * they apply.
  */
 
-#include <stdlib.h>
+#include <unistd.h>
 #include <signal.h>
 #include "exit-value.h"
 

newlib/libc/machine/amdgcn/getreent.c

@@ -3,6 +3,7 @@
 #include <reent.h>
 #include <stdint.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 /* Copied from the HSA documentation.  */
 typedef struct hsa_signal_s {

newlib/libc/sys/amdgcn/fstat.c

@@ -13,6 +13,8 @@
  * they apply.
  */
 
+#include <sys/types.h>
+#include <sys/stat.h>
 #include <unistd.h>
 #include <errno.h>