revert 6dcad60c0ef48af584395a40feeb256fb82986a8 
as it breaks any userland in qemu

diff -Nur gcc-6.3.0.orig/gcc/config/microblaze/microblaze.h gcc-6.3.0/gcc/config/microblaze/microblaze.h
--- gcc-6.3.0.orig/gcc/config/microblaze/microblaze.h	2016-01-21 18:10:54.000000000 +0100
+++ gcc-6.3.0/gcc/config/microblaze/microblaze.h	2017-05-26 18:33:31.297534916 +0200
@@ -253,14 +253,14 @@
 #define FIXED_REGISTERS							\
 {									\
   1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1,			\
-  1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,			\
+  1, 1, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                       \
   1, 1, 1, 1 								\
 }
 
 #define CALL_USED_REGISTERS						\
 {									\
   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,			\
-  1, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,			\
+  1, 1, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                       \
   1, 1, 1, 1								\
 }
 #define GP_REG_FIRST    0