RISC-V: Report unresolved relocation error via linker's callback function.

RISC-V: Report unresolved relocation error via linker's callback function.

Two patches from Nelson Chu.

It is better to use the linker's callback functions to handle the link time
error when relocating.  The unresolved relocation error can be regarded as
an unsupported relocation.  To make user easier to understand different errors,
we need to extend the current error message format of the callback function
since the format is fixed.

	bfd/
	* elfnn-riscv.c (riscv_elf_relocate_section): Use asprintf to extend
	the error message if needed, and then store the result into the
	`msg_buf`.  Finally, remember to free the unused `msg_buf`.  All error
	message for the dangerous relocation should be set before we call the
	callback function.  If we miss the error message since linker runs out
	of memory, we should set the default error message for the error.

	ld/
	* testsuite/ld-riscv-elf/lib-nopic-01a.s: Create the shared library
	lib-nopic-01a.so, it will be linked with lib-nopic-01b.s.
	* testsuite/ld-riscv-elf/lib-nopic-01b.s: Add new test for the
	unresolved relocation.  Link the non-pic code into a shared library
	may cause the error.
	* testsuite/ld-riscv-elf/lib-nopic-01b.d: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Run the new test only when
	the shared library is supported.

R_RISCV_CALL, R_RISCV_JAL and R_RISCV_RVC_JUMP are pc-relative relocation.
For now, we do not allow the object with these relocation links into a shared
library since the referenced symbols may be loaded to the places that too far
from the pc.  We can improve the error message for these unsupported relocation
to notice user that they should recompile their code with `fPIC`.

	bfd/
	* elfnn-riscv.c (riscv_elf_relocate_section): Report the error message
	that user should recompile their code with `fPIC` when linking non-pic
	code into shared library.

	ld/
	* testsuite/ld-riscv-elf/lib-nopic-01b.d: Update the error message.

Change-Id: Ib3347a0a6fa1c2b20a9647c314d5bec2c322ff04