Introduce compiler.h header file

This moves compiler related macros from qemu-common.h to compiler.h.

The reason for this change is that there are simple header files that
depend only on the compiler macros, so including qemu-common.h is overkill.

Besides, qemu-common.h is bloated and will benefit from some splitting.

Please, also note that the QEMU_BUILD_BUG_ON() macro is being fixed to
not use double underscores as a prefix and the license text was added
by Vassili Karpov (malc), who is one of the authors of the new file.

Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>

compiler.h

@@ -0,0 +1,34 @@
+/* public domain */
+
+#ifndef COMPILER_H
+#define COMPILER_H
+
+#include "config-host.h"
+
+#define QEMU_NORETURN __attribute__ ((__noreturn__))
+#ifdef CONFIG_GCC_ATTRIBUTE_WARN_UNUSED_RESULT
+#define QEMU_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+#else
+#define QEMU_WARN_UNUSED_RESULT
+#endif
+
+#define QEMU_BUILD_BUG_ON(x) \
+    typedef char qemu_build_bug_on__##__LINE__[(x)?-1:1];
+
+#if defined __GNUC__
+# if (__GNUC__ < 4) || \
+     defined(__GNUC_MINOR__) && (__GNUC__ == 4) && (__GNUC_MINOR__ < 4)
+   /* gcc versions before 4.4.x don't support gnu_printf, so use printf. */
+#  define GCC_ATTR __attribute__((__unused__, format(printf, 1, 2)))
+#  define GCC_FMT_ATTR(n, m) __attribute__((format(printf, n, m)))
+# else
+   /* Use gnu_printf when supported (qemu uses standard format strings). */
+#  define GCC_ATTR __attribute__((__unused__, format(gnu_printf, 1, 2)))
+#  define GCC_FMT_ATTR(n, m) __attribute__((format(gnu_printf, n, m)))
+# endif
+#else
+#define GCC_ATTR /**/
+#define GCC_FMT_ATTR(n, m)
+#endif
+
+#endif /* COMPILER_H */