arm64: compat: Move sigreturn32.S to .rodata section

Kuser code should be inside .rodata. sigreturn32.S is splited
from kuser32.S, the code in .text section is never executed.

Move it to .rodata.

Signed-off-by: Chen Zhongjin <chenzhongjin@huawei.com>
Link: https://lore.kernel.org/r/20220701035456.250877-1-chenzhongjin@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>