riscv: switch to relative extable and other improvements

riscv: switch to relative extable and other improvements

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.
And recently, arm64 and x86 remove anonymous out-of-line fixups, we
want to acchieve the same result.