riscv: remove DL_RO_DYN_SECTION

While working on enabling D front-end (GDC) in GCC we noticed that
druntime was segfaulting if it is linked dynamically. This was tracked
to DL_RO_DYN_SECTION.

DL_RO_DYN_SECTION lines seem to be copied from MIPS file (which is the
only user of it), but the comment doesn't apply to RISC-V. There is no
such requirement in RISC-V ABI.

        [BZ#24484]
	* sysdeps/riscv/ldsodefs.h: Remove DL_RO_DYN_SECTION as it is not
	required by RISC-V ABI.