powerpc/uaccess: Don't use "m<>" constraint

The "m<>" constraint breaks compilation with GCC 4.6.x era compilers.

The use of the constraint allows the compiler to use update-form
instructions, however in practice current compilers never generate
those forms for any of the current uses of __put_user_asm_goto().

We anticipate that GCC 4.6 will be declared unsupported for building
the kernel in the not too distant future. So for now just switch to
the "m" constraint.

Fixes: 334710b1 ("powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://lore.kernel.org/r/20200507123324.2250024-1-mpe@ellerman.id.au