When building executable, undefined symbol is a fatal error. We don't
complain about -fPIC if the symbol is undefined.
bfd/
PR ld/17847
* elf64-x86-64.c (elf_x86_64_relocate_section): Don't complain
about -fPIC if the symbol is undefined when building executable.
ld/testsuite/
PR ld/17847
* ld-x86-64/pie1.d: New file.
* ld-x86-64/pie1.s: Likwise.
* ld-x86-64/x86-64.exp: Run pie1.