LoongArch: Add 128-bit atomic cmpxchg support

LoongArch: Add 128-bit atomic cmpxchg support

Implement 128-bit atomic compare-and-exchange using LoongArch's
LL.D/SC.Q instructions.

At the same time, this fix the BPF scheduler test failures (scx_central
and scx_qmap) caused by kmalloc_nolock_noprof() returning NULL, due to
missing 128-bit atomics. The NULL returns lead to -ENOMEM errors during
scheduler initialization, causing test cases to fail.

Verified by testing with the scx_qmap scheduler
(located in tools/sched_ext/).

Building with `make` and running ./tools/sched_ext/build/bin/scx_qmap.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=5fb750e8a9ae
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Co-developed-by:: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>