tests: Adjust header inclusions and Makefile.test

This fixes the decrementer, illegal and sc tests to compile
successfully with cross-compilers that don't provide unistd.h
or string.h.  Instead of those headers we include stddef.h.

This also fixes tests/Makefile.test to do what one expects
when CROSS_COMPILE is defined in the environment.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

tests/Makefile.test

@@ -1,7 +1,7 @@
 ARCH = $(shell uname -m)
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
-        CROSS_COMPILE = powerpc64le-linux-
+        CROSS_COMPILE ?= powerpc64le-linux-
         endif
         endif
 

tests/decrementer/decrementer.c

@@ -1,5 +1,4 @@
-#include <unistd.h>
-#include <string.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdbool.h>
 

tests/illegal/illegal.c

@@ -1,5 +1,4 @@
-#include <unistd.h>
-#include <string.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdbool.h>
 

tests/sc/sc.c

@@ -1,5 +1,4 @@
-#include <unistd.h>
-#include <string.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdbool.h>