riscv: compat: vdso: Add COMPAT_VDSO base code implementation

There is no vgettimeofday supported in rv32 that makes simple to
generate rv32 vdso code which only needs riscv64 compiler. Other
architectures need change compiler or -m (machine parameter) to
support vdso32 compiling. If rv32 support vgettimeofday (which
cause C compile) in future, we would add CROSS_COMPILE to support
that makes more requirement on compiler enviornment.

linux-rv64/arch/riscv/kernel/compat_vdso/compat_vdso.so.dbg:
file format elf64-littleriscv

Disassembly of section .text:

0000000000000800 <__vdso_rt_sigreturn>:
800: 08b00893 li a7,139
804: 00000073 ecall
808: 0000 unimp
...

000000000000080c <__vdso_getcpu>:
80c: 0a800893 li a7,168
810: 00000073 ecall
814: 8082 ret
...

0000000000000818 <__vdso_flush_icache>:
818: 10300893 li a7,259
81c: 00000073 ecall
820: 8082 ret

linux-rv32/arch/riscv/kernel/vdso/vdso.so.dbg:
file format elf32-littleriscv

Disassembly of section .text:

00000800 <__vdso_rt_sigreturn>:
800: 08b00893 li a7,139
804: 00000073 ecall
808: 0000 unimp
...

0000080c <__vdso_getcpu>:
80c: 0a800893 li a7,168
810: 00000073 ecall
814: 8082 ret
...

00000818 <__vdso_flush_icache>:
818: 10300893 li a7,259
81c: 00000073 ecall
820: 8082 ret

Finally, reuse all *.S from vdso in compat_vdso that makes
implementation clear and readable.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-17-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>