powerpc/uaccess: Use YZ asm constraint for ld

The 'ld' instruction requires a 4-byte aligned displacement because it
is a DS-form instruction. But the "m" asm constraint doesn't enforce
that.

Add a special case of __get_user_asm2_goto() so that the "YZ" constraint
can be used for "ld".

The "Z" constraint is documented in the GCC manual PowerPC machine
constraints, and specifies a "memory operand accessed with indexed or
indirect addressing". "Y" is not documented in the manual but specifies
a "memory operand for a DS-form instruction". Using both allows the
compiler to generate a DS-form "ld" or X-form "ldx" as appropriate.

The change has to be conditional on CONFIG_PPC_KERNEL_PREFIXED because
the "Y" constraint does not guarantee 4-byte alignment when prefixed
instructions are enabled.

No build errors have been reported due to this, but the possibility is
there depending on compiler code generation decisions.

Fixes: c20beffe ("powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240529123029.146953-2-mpe@ellerman.id.au