riscv: s64ilp32: Add ELF32 support

Use abi_len to distinct ELF32 and ELF64 because s64ilp32 is xlen=64 and
abi_len=32 (__SIZEOF_POINTER__=4). And s64ilp32 is an ELF32 based the
same as s32ilp32.

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