RISC-V/BFD: Correct a missing initializer error with pre-4.7 GCC

RISC-V/BFD: Correct a missing initializer error with pre-4.7 GCC

Correct a commit 9d06997adb88 ("RISC-V: Relax RISCV_PCREL_* to
RISCV_GPREL_*") regression and remove a `-Wmissing-field-initializers'
compilation error:

cc1: warnings being treated as errors
.../bfd/elfnn-riscv.c: In function '_bfd_riscv_relax_pc':
.../bfd/elfnn-riscv.c:3117: error: missing initializer
.../bfd/elfnn-riscv.c:3117: error: (near initialization for 'hi_reloc.hi_addend')
make[4]: *** [elf64-riscv.lo] Error 1

which for versions of GCC before 4.7 prevents support for RISC-V targets
from being built.  See also GCC PR c/36750.

	bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_pc): Use `memset' to
	initialize `hi_reloc'.