powerpc/uaccess: Don't use "m<>" constraint with GCC 4.9

GCC 4.9 sometimes fails to build with "m<>" constraint in
inline assembly.

  CC      lib/iov_iter.o
In file included from ./arch/powerpc/include/asm/cmpxchg.h:6:0,
                 from ./arch/powerpc/include/asm/atomic.h:11,
                 from ./include/linux/atomic.h:7,
                 from ./include/linux/crypto.h:15,
                 from ./include/crypto/hash.h:11,
                 from lib/iov_iter.c:2:
lib/iov_iter.c: In function 'iovec_from_user.part.30':
./arch/powerpc/include/asm/uaccess.h:287:2: error: 'asm' operand has impossible constraints
  __asm__ __volatile__(    \
  ^
./include/linux/compiler.h:78:42: note: in definition of macro 'unlikely'
 # define unlikely(x) __builtin_expect(!!(x), 0)
                                          ^
./arch/powerpc/include/asm/uaccess.h:583:34: note: in expansion of macro 'unsafe_op_wrap'
 #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
                                  ^
./arch/powerpc/include/asm/uaccess.h:329:10: note: in expansion of macro '__get_user_asm'
  case 4: __get_user_asm(x, (u32 __user *)ptr, retval, "lwz"); break; \
          ^
./arch/powerpc/include/asm/uaccess.h:363:3: note: in expansion of macro '__get_user_size_allowed'
   __get_user_size_allowed(__gu_val, __gu_addr, __gu_size, __gu_err); \
   ^
./arch/powerpc/include/asm/uaccess.h:100:2: note: in expansion of macro '__get_user_nocheck'
  __get_user_nocheck((x), (ptr), sizeof(*(ptr)), false)
  ^
./arch/powerpc/include/asm/uaccess.h:583:49: note: in expansion of macro '__get_user_allowed'
 #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
                                                 ^
lib/iov_iter.c:1663:3: note: in expansion of macro 'unsafe_get_user'
   unsafe_get_user(len, &uiov[i].iov_len, uaccess_end);
   ^
make[1]: *** [scripts/Makefile.build:283: lib/iov_iter.o] Error 1

Define a UPD_CONSTR macro that is "<>" by default and
only "" with GCC prior to GCC 5.

Fixes: fcf1f268 ("powerpc/uaccess: Add pre-update addressing to __put_user_asm_goto()")
Fixes: 2f279eeb ("powerpc/uaccess: Add pre-update addressing to __get_user_asm() and __put_user_asm()")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/212d3bc4a52ca71523759517bb9c61f7e477c46a.1603179582.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/asm-const.h

@@ -11,4 +11,17 @@
 #  define __ASM_CONST(x)	x##UL
 #  define ASM_CONST(x)		__ASM_CONST(x)
 #endif
+
+/*
+ * Inline assembly memory constraint
+ *
+ * GCC 4.9 doesn't properly handle pre update memory constraint "m<>"
+ *
+ */
+#if defined(GCC_VERSION) && GCC_VERSION < 50000
+#define UPD_CONSTR ""
+#else
+#define UPD_CONSTR "<>"
+#endif
+
 #endif /* _ASM_POWERPC_ASM_CONST_H */

arch/powerpc/include/asm/uaccess.h

@@ -223,7 +223,7 @@ do {								\
 		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
 		EX_TABLE(1b, %l2)				\
 		:						\
-		: "r" (x), "m<>" (*addr)				\
+		: "r" (x), "m"UPD_CONSTR (*addr)		\
 		:						\
 		: label)
 
@@ -294,7 +294,7 @@ extern long __get_user_bad(void);
 		".previous\n"				\
 		EX_TABLE(1b, 3b)			\
 		: "=r" (err), "=r" (x)			\
-		: "m<>" (*addr), "i" (-EFAULT), "0" (err))
+		: "m"UPD_CONSTR (*addr), "i" (-EFAULT), "0" (err))
 
 #ifdef __powerpc64__
 #define __get_user_asm2(x, addr, err)			\