LoongArch: Add vDSO syscall __vdso_getcpu()

We test 20 million times of getcpu(), the real syscall version take 25
seconds, while the vsyscall version take only 2.4 seconds.

Signed-off-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add VDSO and VSYSCALL support

Add VDSO and VSYSCALL support (sigreturn, gettimeofday and its friends)
for LoongArch.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>