Fix potential conflict of __attribute__((noreturn))

If <stdnoreturn.h> is included first, then noreturn is defined to
_Noreturn, and then __attribute__((noreturn)) will end up being
__attribute__((_Noreturn)), which would not be recognized and might
result in a compiler warning.  To avoid this, use the alternative
spelling __attribute__((__noreturn__)) instead.  This is supported by
the same compiler versions.

Author: petere

perl.h

@@ -438,7 +438,7 @@ Now a no-op.
 #  define __attribute__nonnull__(a)         __attribute__((nonnull(a)))
 #endif
 #ifdef HASATTRIBUTE_NORETURN
-#  define __attribute__noreturn__           __attribute__((noreturn))
+#  define __attribute__noreturn__           __attribute__((__noreturn__))
 #endif
 #ifdef HASATTRIBUTE_PURE
 #  define __attribute__pure__               __attribute__((pure))