RISC-V: Add elfNN_riscv_mkobject to initialize RISC-V tdata.

RISC-V: Add elfNN_riscv_mkobject to initialize RISC-V tdata.

For now we only have one char pointer in RISC-V tdata, so it should be fine.
But once we need more elements in tdata, then we may get some uninitialize
or unexpected values.  I do meet the same problem when extending the RISC-V
tdata.

	bfd/
	elfnn-riscv.c (elfNN_riscv_mkobject):  New function.  We need this
	to initialize RISC-V tdata.