Fix v850 target for RTEMS

Do not define __ATTRIBUTE_IMPURE_PTR__ for RTMES on the v850 target.
The previous definition lead to the following linker error in
combination with -fdata-sections:

relocation truncated to fit: R_V810_GPWLO_1 against symbol
`_global_impure_ptr' defined in .rodata._global_impure_ptr section in
libc.a(lib_a-impure.o)

relocation truncated to fit: R_V810_GPWLO_1 against symbol `_impure_ptr'
defined in .data._impure_ptr section in libc.a(lib_a-impure.o)

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

newlib/libc/include/sys/config.h

@@ -108,7 +108,7 @@
 #define _POINTER_INT short
 #endif
 
-#ifdef __v850
+#if defined(__v850) && !defined(__rtems__)
 #define __ATTRIBUTE_IMPURE_PTR__ __attribute__((__sda__))
 #endif