riscv: vmlinux.lds.S: explicitly catch .rela.dyn symbols

When enabling linker orphan section warning, I got warnings similar as
below:
riscv64-linux-gnu-ld: warning: orphan section `.rela.text' from
`init/main.o' being placed in section `.rela.dyn'

Use the approach similar as ARM64 does and declare it in vmlinux.lds.S

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20230119155417.2600-3-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>