microblaze: Fix put_user macro for 64bits arguments

For 64bits arguments gcc caused that put_user macro
works with wrong value because of optimalization.
Adding volatile caused that gcc not optimized it.

It is possible to use (as Blackfin do) two put_user
macros with 32bits arguments but there is one more
instruction which is due to duplication zero return
value which is called put_user_asm macro.

Signed-off-by: Michal Simek <monstr@monstr.eu>