arm: add get_tls syscall from linux v4.15

for systems without tp register or kuser helper, new in linux commit
8fcd6c45f5a65621ec809b7866a3623e9a01d4ed

arch/arm/bits/syscall.h.in

@@ -359,4 +359,5 @@
 #define __ARM_NR_usr26		0x0f0003
 #define __ARM_NR_usr32		0x0f0004
 #define __ARM_NR_set_tls	0x0f0005
+#define __ARM_NR_get_tls	0x0f0006