riscv: switch to relative exception tables

Similar as other architectures such as arm64, x86 and so on, use
offsets relative to the exception table entry values rather than
absolute addresses for both the exception locationand the fixup.

However, RISCV label difference will actually produce two relocations,
a pair of R_RISCV_ADD32 and R_RISCV_SUB32. Take below simple code for
example:

$ cat test.S
.section .text
1:
        nop
.section __ex_table,"a"
        .balign 4
        .long (1b - .)
.previous

$ riscv64-linux-gnu-gcc -c test.S
$ riscv64-linux-gnu-readelf -r test.o
Relocation section '.rela__ex_table' at offset 0x100 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000000  000600000023 R_RISCV_ADD32     0000000000000000 .L1^B1 + 0
000000000000  000500000027 R_RISCV_SUB32     0000000000000000 .L0  + 0

The modpost will complain the R_RISCV_SUB32 relocation, so we need to
patch modpost.c to skip this relocation for .rela__ex_table section.

After this patch, the __ex_table section size of defconfig vmlinux is
reduced from 7072 Bytes to 3536 Bytes.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>