arm64: Optimize __READ_ONCE() with CONFIG_LTO=y

Rework arm64 LTO __READ_ONCE() to improve code generation as follows:

1. Replace _Generic-based __unqual_scalar_typeof() with more complete
__rwonce_typeof_unqual(). This strips qualifiers from all types, not
just integer types, which is required to be able to assign (must be
non-const) to __u.__val in the non-atomic case (required for #2).

One subtle point here is that non-integer types of __val could be const
or volatile within the union with the old __unqual_scalar_typeof(), if
the passed variable is const or volatile. This would then result in a
forced load from the stack if __u.__val is volatile; in the case of
const, it does look odd if the underlying storage changes, but the
compiler is told said member is "const" -- it smells like UB.

2. Eliminate the atomic flag and ternary conditional expression. Move
the fallback volatile load into the default case of the switch,
ensuring __u is unconditionally initialized across all paths.
The statement expression now unconditionally returns __u.__val.

This refactoring appears to help the compiler improve (or fix) code
generation.

With a defconfig + LTO + debug options builds, we observe different
codegen for the following functions:

btrfs_reclaim_sweep (708 -> 1032 bytes)
btrfs_sinfo_bg_reclaim_threshold_store (200 -> 204 bytes)
check_mem_access (3652 -> 3692 bytes) [inlined bpf_map_is_rdonly]
console_flush_all (1268 -> 1264 bytes)
console_lock_spinning_disable_and_check (180 -> 176 bytes)
igb_add_filter (640 -> 636 bytes)
igb_config_tx_modes (2404 -> 2400 bytes)
kvm_vcpu_on_spin (480 -> 476 bytes)
map_freeze (376 -> 380 bytes)
netlink_bind (1664 -> 1656 bytes)
nmi_cpu_backtrace (404 -> 400 bytes)
set_rps_cpu (516 -> 520 bytes)
swap_cluster_readahead (944 -> 932 bytes)
tcp_accecn_third_ack (328 -> 336 bytes)
tcp_create_openreq_child (1764 -> 1772 bytes)
tcp_data_queue (5784 -> 5892 bytes)
tcp_ecn_rcv_synack (620 -> 628 bytes)
xen_manage_runstate_time (944 -> 896 bytes)
xen_steal_clock (340 -> 296 bytes)

Increase of some functions are due to more aggressive inlining due to
better codegen (in this build, e.g. bpf_map_is_rdonly is no longer
present due to being inlined completely).

NOTE: The return-value-of-function-drops-qualifiers hack was first
suggested by Al Viro in [1], which notes some of its limitations which
make it unsuitable for a general __unqual_scalar_typeof() replacement.
Notably, array types are not supported, and GCC 8.1-8.3 still fail. Why
should we use it here? READ_ONCE() does not support reading whole
arrays, and the GCC version problem only affects 3 minor releases of a
very ancient still-supported GCC version; not only that, this arm64
READ_ONCE() version is currently only activated by LTO builds, which
to-date are *only supported by Clang*!

Link: https://lore.kernel.org/all/20260111182010.GH3634291@ZenIV/ [1]
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Will Deacon <will@kernel.org>