RISC-V: Don't do undefweak relaxations for the linker_def symbols.

RISC-V: Don't do undefweak relaxations for the linker_def symbols.

I get the following truncated errors recently when running riscv-gnu-toolchain
regressions,

/scratch/riscv-gnu-toolchain/regression/build/linux-rv32imafdc-ilp32d-medlow/build-glibc-linux-rv32imafdc-ilp32d/libc.a(libc-start.o): in function `elf_irela':
/scratch/riscv-gnu-toolchain/glibc/csu/../sysdeps/riscv/dl-irel.h:47:(.text+0x88): relocation truncated to fit: R_RISCV_GPREL_I against symbol `__ehdr_start' defined in .note.ABI-tag section in /scratch/riscv-gnu-toolchain/regression/build/linux-rv32imafdc-ilp32d-medlow/build-glibc-linux-rv32imafdc-ilp32d/elf/sln

The linker_def symbols like __ehdr_start that may be undefweak in early stages
of linking, including relax stage, but are guaranteed to be defined later.
Therefore, it seems like we shouldn't do the undefweak relaxations for these
kinds of symbols since they may be defined after relaxations.

bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_section): Don't do undefweak
	relaxations for the linker_def symbols.