target/ppc: Migrate atomic loads to decodetree

Migrate load-and-reserve instructions (lbarx, lharx, lwarx, ldarx,
lqarx) to decodetree using the X-form layout.

A shared helper (do_load_locked) is introduced for standard-width
variants, while LQARX is handled separately due to its 128-bit
semantics and register pairing constraints.

The implementation preserves legacy behavior, including:
  - Reservation granularity and overwrite semantics
  - Alignment requirements
  - Invalid instruction cases for LQARX

Testing:
  - Verified TCG equivalence with legacy implementation

Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
Reviewed-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Tested-by: Aniket Sahu <asahu1x@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260827133010.278889-3-rathc@linux.ibm.com
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>