LoongArch: Add HWCAP_LOONGARCH_CPUCFG to elf_hwcap

HWCAP_LOONGARCH_CPUCFG is missing in elf_hwcap, so add it for glibc's
later use.

Cc: stable@vger.kernel.org
Reported-by: Yinyu Cai <caiyinyu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>