s390/jump_label: use "i" constraint for clang

Currently kernel build fails under clang if jump labels are enabled.
The problem is "X" constraint usage "Any operand whatsoever is allowed",
for which clang produces the following:
.pushsection __jump_table,"aw"
.balign 8
.long   0b-.,.Ltmp577-.
.quad   %r0+0-. # %r0 is not allowed here
.popsection

Under gcc constraints "X" or "jdd" (gcc > 9) are used for static keys.
Ideally, we'd have used "i" for gcc, but it doesn't work in all cases
with -fPIC code. This is gcc-specific problem that doesn't exist in llvm.
Since clang does not have "jdd" simply always use "i" constraint for it.

Suggested-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/jump_label.h

@@ -10,7 +10,9 @@
 #define JUMP_LABEL_NOP_SIZE 6
 #define JUMP_LABEL_NOP_OFFSET 2
 
-#if __GNUC__ < 9
+#ifdef CONFIG_CC_IS_CLANG
+#define JUMP_LABEL_STATIC_KEY_CONSTRAINT "i"
+#elif __GNUC__ < 9
 #define JUMP_LABEL_STATIC_KEY_CONSTRAINT "X"
 #else
 #define JUMP_LABEL_STATIC_KEY_CONSTRAINT "jdd"