fix microblaze syscall register clobbers

the kernel entry point for syscalls on microblaze nominally saves and
restores all registers, and testing on qemu always worked since qemu
behaves this way too. however, the real kernel treats r3:r4 as a
potential 64-bit return value from the syscall function, and copies
both over top of the saved registers before returning to userspace.
thus, we need to treat r4 as always-clobbered.

arch/microblaze/syscall_arch.h

@@ -13,7 +13,7 @@ static __inline long __syscall0(long n)
 	register unsigned long r3 __asm__("r3");
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -24,7 +24,7 @@ static inline long __syscall1(long n, long a)
 	register unsigned long r5 __asm__("r5") = a;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -36,7 +36,7 @@ static inline long __syscall2(long n, long a, long b)
 	register unsigned long r6 __asm__("r6") = b;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5), "r"(r6)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -49,7 +49,7 @@ static inline long __syscall3(long n, long a, long b, long c)
 	register unsigned long r7 __asm__("r7") = c;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5), "r"(r6), "r"(r7)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -63,7 +63,7 @@ static inline long __syscall4(long n, long a, long b, long c, long d)
 	register unsigned long r8 __asm__("r8") = d;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5), "r"(r6), "r"(r7), "r"(r8)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -78,7 +78,7 @@ static inline long __syscall5(long n, long a, long b, long c, long d, long e)
 	register unsigned long r9 __asm__("r9") = e;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5), "r"(r6), "r"(r7), "r"(r8), "r"(r9)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }
 
@@ -94,7 +94,7 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	register unsigned long r10 __asm__("r10") = f;
 	__asm__ __volatile__ ("brki r14, 0x8" : "=r"(r3)
 		: "r"(r12), "r"(r5), "r"(r6), "r"(r7), "r"(r8), "r"(r9), "r"(r10)
-		: "memory");
+		: "memory", "r4");
 	return r3;
 }