powerpc/atomic: Use YZ constraints for DS-form instructions

The 'ld' and 'std' instructions require a 4-byte aligned displacement
because they are DS-form instructions. But the "m" asm constraint
doesn't enforce that.

That can lead to build errors if the compiler chooses a non-aligned
displacement, as seen with GCC 14:

  /tmp/ccuSzwiR.s: Assembler messages:
  /tmp/ccuSzwiR.s:2579: Error: operand out of domain (39 is not a multiple of 4)
  make[5]: *** [scripts/Makefile.build:229: net/core/page_pool.o] Error 1

Dumping the generated assembler shows:

  ld 8,39(8)       # MEM[(const struct atomic64_t *)_29].counter, t

Use the YZ constraints to tell the compiler either to generate a DS-form
displacement, or use an X-form instruction, either of which prevents the
build error.

See commit 2d43cc701b96 ("powerpc/uaccess: Fix build errors seen with
GCC 13/14") for more details on the constraint letters.

Fixes: 9f0cbea0d8cc ("[POWERPC] Implement atomic{, 64}_{read, write}() without volatile")
Cc: [email protected] # v2.6.24+
Reported-by: Stephen Rothwell <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Tested-by: Mina Almasry <[email protected]>
Reviewed-by: Segher Boessenkool <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

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