s390x: shuffle register usage in __tls_get_offset to avoid r0 as address

This fixes an error in 6af4f25b899e89e4b91f8c197ae5a6ce04bcce7b: The
r0 register is special in addressing modes on s390x and is interpreted
as constant zero, i.e. lg %r5, 8(%r0) would effectively become lg %r5,
8. So care should be taken to never use r0 as an address register in
s390x assembly.

src/thread/s390x/__tls_get_offset.s

@@ -1,17 +1,17 @@
 	.global __tls_get_offset
 	.type __tls_get_offset,%function
 __tls_get_offset:
-	ear   %r0, %a0
-	sllg  %r0, %r0, 32
-	ear   %r0, %a1
+	ear   %r3, %a0
+	sllg  %r3, %r3, 32
+	ear   %r3, %a1
 
 	la    %r1, 0(%r2, %r12)
 
-	lg    %r3, 0(%r1)
-	sllg  %r4, %r3, 3
-	lg    %r5, 8(%r0)
+	lg    %r0, 0(%r1)
+	sllg  %r4, %r0, 3
+	lg    %r5, 8(%r3)
 	lg    %r2, 0(%r4, %r5)
 	ag    %r2, 8(%r1)
-	sgr   %r2, %r0
+	sgr   %r2, %r3
 
 	br    %r14