LoongArch: Detect and expose DIV32 feature existence

The LoongArch Architecture Manual defines a hardware capability bit
CPUCFG2_DIV32 (bit 26 of CPUCFG 0x2). When this bit is set to 1, it
indicates that on 64-bit machines, the division and mod instructions
(DIV.W[U] and MOD.W[U]) will ignore the upper 32 bits of the input
registers and only compute based on the lower 32 bits.

On certain CPU cores without this feature, these 32-bit instructions
require both input operands to be properly sign-extended. Otherwise,
the hardware execution results can be randomly meaningless values.

Add the CPU_FEATURE_DIV32 definition, detect and export this hardware
capability via elf_hwcap and cpu_has_div32 (in cpuinfo). This provides
the infrastructure for subsystems like BPF JIT to safely choose either
native 32-bit instructions or fallback instruction sequences based on
the actual CPU hardware capabilities.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

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