riscv: s64ilp32: Add MMU_SV39 mode support

There is no MMU_SV32 support in xlen=64 ISA generally, but s64ilp32
selects 32BIT, which uses MMU_SV32 default. This commit enables MMU_SV39
for 32BIT to satisfy the 4GB mapping requirement. The Sv39 is the
mandatory MMU mode in RVA20S64 and RVA22S64, so we needn't care about
Sv48 & Sv57.

We use duplicate remapping to solve the address sign extension problem
from the compiler. Make the address of 0xffffffff80000000 equal to
0x80000000 by pg_dir[2] = pg_dir[510] and pg_dir[3] = pg_dir[511] of the
page table.

Why didn't we prevent address sign extension in the compiler?
 - Additional zero extension reduces the performance
 - Prevent complex and unnecessary work for compiler guys.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>