RISC-V: Force linker error exit after unresolvable reloc.

RISC-V: Force linker error exit after unresolvable reloc.

This was noticed while trying to test the compiler -msave-restore support.
Putting non-pic code in a shared library gives a linker error, but doesn't
stop the build.

rohan:2030$ cat libtmp.c
extern int sub2 (int);
int sub (int i) { return sub2 (i + 10); }
rohan:2031$ cat libtmp2.c
extern int sub (int);
int sub2 (int i) { return sub (i + 10); }
rohan:2032$ riscv64-unknown-linux-gnu-gcc --shared -o libtmp.so libtmp.c
rohan:2033$ riscv64-unknown-linux-gnu-gcc --shared -o libtmp2.so libtmp2.c libtmp.so
/home/jimw/FOSS/install-riscv64/lib/gcc/riscv64-unknown-linux-gnu/8.3.0/../../../../riscv64-unknown-linux-gnu/bin/ld: /tmp/cctrsIBe.o(.text+0x18): unresolvable R_RISCV_CALL relocation against symbol `sub'
rohan:2034$ echo $?
0
rohan:2035$ ls -lt libtmp2.so
-rwxr-xr-x 1 jimw jimw 6912 Aug 30 14:32 libtmp2.so
rohan:2036$

The patch fixes this by forcing a linker error.  I now get this.

ohan:2059$ sh tmp.script
/home/jimw/FOSS/BINUTILS/X-riscv64-linux/ld/ld-new: libtmp2.o(.text+0x18): unresolvable R_RISCV_CALL relocation against symbol `sub'
/home/jimw/FOSS/BINUTILS/X-riscv64-linux/ld/ld-new: final link failed: bad value
rohan:2060$ echo $?
1
rohan:2061$ ls -lt libtmp2.so
ls: cannot access 'libtmp2.so': No such file or directory

	bfd/
	* elfnn-riscv.c (riscv_elf_relocate_section): For unresolvable reloc
	error, call bfd_set_error, set ret to FALSE, and goto out label.