[SPARC/RTEMS] Add __FIX_LEON3FT_B2BST

In case the LEON3FT back-to-back store workaround is active
(sparc_fix_b2bst), then define the builtin define __FIX_LEON3FT_B2BST on
RTEMS.  The intended use case for this is operating system code in
assembly language.  See also:

https://lists.rtems.org/pipermail/devel/2017-July/018463.html

gcc/
	*  gcc/config/sparc/rtemself.h (TARGET_OS_CPP_BUILTINS): Add
	conditional builtin define __FIX_LEON3FT_B2BST.

From-SVN: r250254

gcc/ChangeLog

+2017-07-17  Sebastian Huber  <sebastian.huber@embedded-brains.de>
+
+	*  gcc/config/sparc/rtemself.h (TARGET_OS_CPP_BUILTINS): Add
+	conditional builtin define __FIX_LEON3FT_B2BST.
+
 2017-07-17  Daniel Cederman  <cederman@gaisler.com>
 
 	* config/sparc/t-rtems: Add mfix-gr712rc multilibs. Replace

gcc/config/sparc/rtemself.h

@@ -26,6 +26,8 @@ along with GCC; see the file COPYING3.  If not see
 	builtin_define ("__rtems__");		\
 	builtin_define ("__USE_INIT_FINI__");	\
 	builtin_assert ("system=rtems");	\
+	if (sparc_fix_b2bst)			\
+	  builtin_define ("__FIX_LEON3FT_B2BST"); \
     }						\
   while (0)